# Solutions to Review Problems

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1 Chapter 1 Solutions to Review Problems Chapter 1 Exercise 42 Which of the following equations are not linear and why: (a x x 2 2x 3 = 5. (b x 1 + x 1 x 2 + 2x 3 = 1. (c x x 2 + x 3 = 5. (a The given equation is linear by (??. (b The equation is not linear because of the term x 1 x 2. (c The equation is nonlinear because x 2 has a negative power Exercise 43 Show that (2s + 12t + 13, s, s 3t 3, t is a solution to the system { 2x1 + 5x 2 + 9x 3 + 3x 4 = 1 x 1 + 2x 2 + 4x 3 = 1 Substituting these values for x 1, x 2, x 3, and x 4 in each equation. 2x 1 + 5x 2 + 9x 3 + 3x 4 = 2(2s + 12t s + 9( s 3t 3 + 3t = 1 x 1 + 2x 2 + 4x 3 = (2s + 12t s + 4( s 3t 3 = 1. Since both equations are satisfied, then it is a solution for all s and t 1

2 2 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 44 Solve each of the following systems using the method of elimination: (a { 4x1 3x 2 = 0 2x 1 + 3x 2 = 18 (b { 4x1 6x 2 = 10 6x 1 9x 2 = 15 (c { 2x1 + x 2 = 3 2x 1 + x 2 = 1 Which of the above systems is consistent and which is inconsistent? (a Adding the two equations to obtain 6x 1 = 18 or x 1 = 3. Substituting this value for x 1 in one of the given equations and then solving for x 2 we find x 2 = 4. So system is consistent. (b The augmented matrix of the given system is ( Divide the first row by 4 to obtain ( Now, add to the second row 6 times the first row to obtain ( Hence, x 2 = s is a free variable. Solving for x 1 we find x 1 = 5+3t.The system 2 is consistent. (c Note that according to the given equation 1 = 3 which is impossible. So the given system is inconsistent

3 3 Exercise 45 Find the general solution of the linear system { x1 2x 2 + 3x 3 + x 4 = 3 2x 1 x 2 + 3x 3 x 4 = 0 The augmented matrix of the given system is ( A corresponding row-echelon matrix is obtained by adding negative two times the first row to the second row. ( Thus x 3 = s and x 4 = t are free variables. Solving for the leading variables one finds x 1 = 1 s + t and x 2 = 2 + s + t Exercise 46 Find a, b, and c so that the system x 1 + ax 2 + cx 3 = 0 bx 1 + cx 2 3x 3 = 1 ax 1 + 2x 2 + bx 3 = 5 has the solution x 1 = 3, x 2 = 1, x 3 = 2. Simply substitute these values into the given system to obtain a + 2c = 3 3b c = 7 3a + 2b = 7 The augmented matrix of the system is

4 4 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS A row-echelon form of this matrix is obtained as follows. Step 1: r 1 r Step 2: r 3 r 3 3r 1 Step 3: r 2 r 2 r 3 Step 4: r 3 r 3 2r Step 5: r r 3 The corresponding system is a 2c = 3 b 7c = 9 c = 1 Using back substitution we find the solution a = 1, b = 2, c = 1 Exercise 47 Find a relationship between a,b,c so that the following system is consistent x 1 + x 2 + 2x 3 = a x 1 + x 3 = b 2x 1 + x 2 + 3x 3 = c

5 5 The augmented matrix of the system is a b c We reduce this matrix into row-echelon form as follows. Step 1: r 2 r 2 r 1 and r 3 r 3 2r a b a c 2a Step 2: r 2 r a a b c 2a Step 3: r 3 r 3 + r a a b c a b The system is consistent provided that a + b c = 0 Exercise 48 For which values of a will the following system have (a no solutions? (b exactly one solution? (c infinitely many solutions? The augmented matrix is x 1 + 2x 2 3x 3 = 4 3x 1 x 2 + 5x 3 = 2 4x 1 + x 2 + (a 2 14x 3 = a a 2 14 a + 2

6 6 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS The reduction of this matrix to row-echelon form is outlined below. Step 1: r 2 r 2 3r 1 and r 3 r 3 4r a 2 2 a 14 Step 2: r 3 r 3 r a 2 16 a 4 The corresponding system is x 1 + 2x 2 3x 3 = 4 7x x 3 = 10 (a 2 16x 3 = a 4 (a If a = 4 then the last equation becomes 0 = 8 which is impossible. Therefore, the system is inconsistent. (b If a ±4 then the system has exactly one solution, namely, x 1 = 8a+15, x 7(a+4 2 = 10a+54, x 7(a+4 3 = 1. a+4 (c If a = 4 then the system has infinitely many solutions. In this case, x 3 = t is the free variable and x 1 = 8 7t and x 7 2 = 10+14t 7 Exercise 49 Find the values of A,B,C in the following partial fraction x 2 x + 3 (x 2 + 2(2x 1 = Ax + B x C 2x 1. By multiplying both sides of the equation by (x 2 + 2(2x 1 and then equating coefficients of like powers of x we obtain the following system 2A + C = 1 A + 2B = 1 B + 2C = 3

7 7 Replace r 3 by 2r 3 + r 2 to obtain 2A + C = 1 A + 2B = 1 A + 4C = 5 Next, replace r 3 by 2r 3 + r 1 to obtain 2A + C = 1 A + 2B = 1 9C = 11 Solving backward, we find A = 1 9, B = 5 9, and C = 11 9 Exercise 50 Find a quadratic equation of the form y = ax 2 + bx + c that goes through the points ( 2, 20, (1, 5, and (3, 25. The components of these points satisfy the given quadratic equation. This leads to the following system Apply Gauss algorithm as follows. 4a 2b + c = 20 a + b + c = 5 9a + 3b + c = 25 Step1. r 1 r 2 a + b + c = 5 4a 2b + c = 20 9a + 3b + c = 25 Step 2. r 2 r 2 4r 1 and r 3 r 3 9r 1 a + b + c = 5 6b 3c = 0 6b 8c = 20

8 8 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Step 3. r 3 r 3 r 2 a + b + c = 5 6b 3c = 0 5c = 20 Solving by the method of backward substitution we find a = 3, b = 2, and c = 4 Exercise 51 For which value(s of the constant k does the following system have (a no solutions? (b exactly one solution? (c infinitely many solutions? { x1 x 2 = 3 2x 1 2x 2 = k (a The system has no solutions if k 3, i.e. k 6. 2 (b The system has no unique solution for any value of k. (c The system has infinitely many solution if k = 6. The general solution is given by x 1 = 3 + t, x 2 = t Exercise 52 Find a linear equation in the unknowns x 1 and x 2 that has a general solution x 1 = 5 + 2t, x 2 = t. Since x 2 = t then x 1 = 5 + 2x 2 that is x 1 2x 2 = 5 Exercise 53 Consider the linear system 2x 1 + 3x 2 4x 3 + x 4 = 5 2x 1 + x 3 = 7 3x 1 + 2x 2 4x 3 = 3 (a Find the coefficient and augmented matrices of the linear system. (b Find the matrix notation.

9 9 (a If A is the coefficient matrix and B is the augmented matrix then A = , B = (b The given system can be written in matrix form as follows x 1 x 2 x 3 = Exercise 54 Solve the following system using elementary row operations on the augmented matrix: 5x 1 5x 2 15x 3 = 40 4x 1 2x 2 6x 3 = 19 3x 1 6x 2 17x 3 = 41 The augmented matrix of the system is The reduction of this matrix to row-echelon form is Step 1: r r Step 2: r 2 r 2 4r 1 and r 3 r 3 3r

10 10 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Step 3: r 2 r 2 + r 3 Step 4: r 3 r 3 3r Step 5: r 2 r 2 and r 3 1r It follows that x 3 = 5 2, x 2 = 6, and x 1 = 11 2 Exercise 55 Solve the following system. 2x 1 + x 2 + x 3 = 1 x 1 + 2x 2 + x 3 = 0 3x 1 2x 3 = 5 The augmented matrix is given by The reduction of the augmented matrix to row-echelon form is as follows. Step 1: r 1 r Step 2: r 2 r 2 2r 1 and r 3 r 3 3r

11 11 Step 3: r 3 r 3 2r Step 4: r 2 1r 3 2 and r 3 1r The solution is given by: x 1 = 1 9, x 2 = 10 9, x 3 = 7 3 Exercise 56 Which of the following matrices are not in reduced row-ehelon from and why? (a (b (c (a No, because the matrix fails condition 1 of the definition. Rows of zeros must be at the bottom of the matrix. (b No, because the matrix fails condition 2 of the definition. Leading entry in row 2 must be 1 and not 2. (c Yes. The given matrix satisfies conditions 1-4 Exercise 57 Use Gaussian elimination to convert the following matrix into a row-echelon

12 12 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS matrix We follow the steps in Gauss-Jordan algorithm. Step 1: r 2 r 2 + r 1, r 3 r 3 2r 1, and r 4 r 4 + r Step 2: r 3 r 3 r 2 and r 4 r 4 = 2r Step 3: r r Step 4: r 4 r 4 + r Exercise 58 Use Gauss-Jordan elimination to convert the following matrix into reduced row-echelon form

13 Using the Gauss-Jordan to bring the given matrix into reduced row-echelon form as follows. 13 Step 1: r 1 r Step 2: r 2 r 2 6r 1, r 3 r 3 + 2r 1, and r 4 r 4 + 5r Step 3: r 2 r Step 4: r 3 r 3 5r 2 and r 4 r r 2 Step 5: r 3 r 3 and r 4 1r Step 6: r 1 r 1 + r

14 14 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Step 7: r 2 r 2 r Step 8: r 1 r 1 + 5r 4 and r 2 r 2 6r Exercise 59 Solve the following system using Gauss-Jordan elimination. 3x 1 + x 2 + 7x 3 + 2x 4 = 13 2x 1 4x x 3 x 4 = 10 5x x 2 7x 3 + 8x 4 = 59 2x 1 + 5x 2 4x 3 3x 4 = 39 The augmented matrix of the system is The reduction of the augmented matrix into reduced row-echelon form is Step 1: r 1 r 1 r Step 2: r 2 r 2 2r 1, r 3 r 3 5r 1, and r 4 r 4 2r

15 15 Step 3: r 3 r 3 r Step 4: r 4 r 3 and r 2 1 r Step 5: r 1 r 1 5r 2 and r 3 r 3 + 5r 2 Step 6: r r Step 7: r 1 r 1.5r 3 and r 2 r 2.5r r The corresponding system is given by x 1 + 3x 3 = 4 x 2 2x 3 = 5 x 4 = 2 The general solution is: x 1 = 4 3t, x 2 = 5 + 2t, x 3 = t, x 4 = 2

16 16 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 60 Find the rank of each of the following matrices. (a (b (a We reduce the given matrix to row-echelon form. Step 1: r 3 r 3 + 4r 1 and r 4 r 4 + 3r Step 2: r 4 r 4 r Step 3: r 1 r 1 and r 2 r Step 4: r 4 r 3 r

17 17 Step 5: r r 2 and r r Thus, the rank of the given matrix is 3. (b Apply the Gauss algorithm as follows. Step 1: r 2 r 2 2r 1 and r 3 r 3 + r Step 2: r 3 r 3 + 2r 2 Step 3: r r 2 Hence, the rank is Exercise 61 Choose h and k such that the following system has (a no solutions, (b exactly one solution, and (c infinitely many solutions. { x1 3x 2 = 1 2x 1 hx 2 = k The augmented matrix of the system is ( h k

18 18 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS By performing the operation r 2 r 2 2r 1 we find ( h k 2 (a The system is inconsistent if h = 6 and k 2. (b The system has exactly one solution if h 6 and for any k. The solution is given by the formula:x 1 = 3k h, x 6 h 2 = k 2. 6 h (c The system has infinitely many solutions if h = 6 and k = 2. The parametric form of the solution is: x 1 = 1 + 3s, x 2 = s Exercise 62 Solve the linear system whose augmented matrix is reduced to the following reduced row-echelon form The free variable is x 4 = s. Solving by back-substitution one finds x 1 = 8 + 7s, x 2 = 2 3s, and x 3 = 5 s Exercise 63 Solve the linear system whose augmented matrix is reduced to the following row-echelon form Because of the last row the system is inconsistent Exercise 64 Solve the linear system whose augmented matrix is given by

19 19 The reduction of the augmented matrix to row-echelon form is Step 1: r 2 r 2 + r 1 and r 3 r 3 3r Step 2: r 2 r 2 Step 3: r 3 r r Step 4: r r Using backward substitution we find the solution: x 1 = 3, x 2 = 1, x 3 = 2 Exercise 65 Find the value(s of a for which the following system has a nontrivial solution. Find the general solution. x 1 + 2x 2 + x 3 = 0 x 1 + 3x 2 + 6x 3 = 0 2x 1 + 3x 2 + ax 3 = 0 The augmented matrix of the system is a 0

20 20 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Reducing this matrix to row-echelon form as follows. Step 1: r 2 r 2 r 1 and r 3 r 3 2r a 2 0 Step 2: r 3 r 3 + r a If a = 3 then the rank of the coefficient matrix is less than the number of unknowns. Therefore, by Theorem 6 the system has a nontrivial solution and consequently infinitely many solutions. By letting x 3 = s we find x 1 = 9s and x 2 = 5s Exercise 66 Solve the following homogeneous system. x 1 x 2 + 2x 3 + x 4 = 0 2x 1 + 2x 2 x 4 = 0 3x 1 + x 2 + 2x 3 + x 4 = 0 The augmented matrix of the system is Reducing the augmented matrix to row-echelon form. Step 1: r 2 r 2 2r 1 and r 3 r 3 3r

21 21 Step 2: r 3 r 3 r Step 3: r r It follows that the rank of the coefficient matrix is less than the number of unknowns so the system has infinitely many solutions given by the formula x 1 = s, x 2 = s, x 3 = s, and x 4 = 0 Exercise 67 Let A be an m n matrix. (a Prove that if y and z are solutions to the homogeneous system Ax = 0 then y + z and cy are also solutions, where c is a number. (b Give a counterexample to show that the above is false for nonhomogeneous systems. (a Suppose that Ax = Ay = 0 and c R then A(x + y = Ax = Ay = 0 and A(cx = cax = 0. (b The ordered pair (1, 0 satisfies the nonhomogeneous system { x1 + x 2 = 1 However, 2(1, 0 is not a solution x 1 x 2 = 1 Exercise 68 Show that the converse of Theorem?? is false. That is, show the existence of a nontrivial solution does not imply that the number of unknowns is greater than the number of equations. Consider the system x 1 + x 2 = 0 2x 1 + 2x 2 = 0 3x 1 + 3x 2 = 0 The system consists of three lines that coincide

22 22 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 69 (Network Flow The junction rule of a network says that at each junction in the network the total flow into the junction must equal the total flows out. To illustrate the use of this rule, consider the network shown in the accompanying diagram. Find the possible flows in the network. Equating the flow in with the flow out at each intersection, we obtain the following system of four equations in the unknowns f 1, f 2, f 3, f 4, and f 5. f 1 f 3 + f 4 = 40(IntersectionA f 1 + f 2 = 50(IntersectionB f 2 + f 3 + f 5 = 60(IntersectionC f 4 + f 5 = 50(IntersectionD The augmented matrix of the system is The reduction of this system to row-echelon form is carried out as follows. Step 1: r 2 r 2 r 1 Step 2: r 3 r 3 r 2 Step 3: r 4 r 4 r

23 This shows that f 3 and f 5 play the role of parameters. Now, solving for the remaining variables we find the general solution of the system which gives all the possible flows Chapter 2 f 1 = f 3 + f 5 10 f 2 = f 3 f f 4 = 50 f 5 23 Exercise 108 Compute the matrix ( T ( ( T ( ( 4 1 = 1 6 Exercise 109 Find w, x, y, and z. 1 2 w 2 x 4 y 4 z = Equating the corresponding entries we find w = 1, x = 3, y = 0, and z = 5 Exercise 110 Determine two numbers s and t such that the following matrix is symmetric. 2 s t A = 2s 0 s + t 3 3 t

24 24 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Since A is symmetric then A T = A; that is, 2 2s 3 A T = s 0 3 = t s + t t 2 s t 2s 0 s + t 3 3 t Equating corresponding entries we find that s = 0 and t = 3 Exercise 111 Let A be a 2 2 matrix. Show that ( ( A = a + b ( c 1 0 ( d 0 1 Let A be the matrix Then ( ( a + b ( 0 0 ( 0 0 a 0 0 b ( 0 a b c ( a b A = c d ( c ( c 0 = A d + ( d 0 1 ( d = = Exercise 112 Let A = [ ], B = [ ], C = [ ]. If ra+sb +tc = 0 show that s = r = t = 0. A simple arithmetic yields the matrix ra + sb + tc = [ r + 3t r + s r + 2s + t ] The condition ra + sb + tc = 0 yields the system r + 3t = 0 r + s = 0 r + 2s + t = 0

25 25 The augmented matrix is Step 1: r 2 r 2 r 1 and r 3 r 3 + r Step 2: r 3 r 3 2r Solving the corresponding system we find r = s = t = 0 Exercise 113 Show that the product of two diagonal matrices is again a diagonal matrix. Let D = (d ij be a diagonal matrix of size m n and D = (d ij be an n p diagonal matrix. Let DD = (a ij. Then a ij = n k=1 d ikd kj. If i = j then a ii = d ii d ii since for i k we have d ik = d ki = 0. Now, if i j then d ikd kj = 0 for all possible values of k. Hence, DD is diagonal Exercise 114 Let A be an arbitrary matrix. Under what conditions is the product AA T defined? Suppose A is an m n matrix. Then A T is an n m matrix. Since the number of columns of A is equal to the number of rows of A T then AA T is always defined Exercise 115 (a Show that AB = BA if and only if (A B(A + B = A 2 B 2. (b Show that AB = BA if and only if (A + B 2 = A 2 + 2AB + B 2.

26 26 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS (a Suppose that AB = BA then (A B(A+B = A 2 +AB BA B 2 = A 2 B 2 since AB BA = 0. Conversely, suppose that (A B(A + B = A 2 B 2 then by expanding the left-hand side we obtain A 2 AB+BA B 2 = A 2 B 2. This implies that AB = BA. (b Suppose that AB = BA then (A + B 2 = A 2 + AB + BA + B 2 = A 2 + 2AB + B 2. Conversely, suppose that (A + B 2 = A 2 + 2AB + B 2. By expanding the left-hand side we obtain A 2 +AB +BA+B 2 = A 2 +2AB +B 2 and this leads to AB = BA Exercise 116 Let A be a matrix of size m n. Denote the columns of A by C 1, C 2,, C n. Let x be the n 1 matrix with entries x 1, x 2,, x n. Show that Ax = x 1 C 1 + x 2 C x n C n. Suppose that A = (a ij. Then a 11 a 12 a 1n x 1 a 11 x 1 + a 12 x a 1n x n a 21 a 22 a 2n x 2 Ax =.... = a 21 x 1 + a 22 x a 2n x n. a m1 a mn a mn x n a m1 x 1 + a m2 x a mn x n a 11 a 12 a 1n a 21 = x 1. + x a x a 2n n. a m1 a m2 = x 1 C 1 + x 2 C x n C n a mn Exercise 117 Let A be an m n matrix. Show that if ya = 0 for all y R m then A = 0. Suppose the contrary. Then there exist indices i and j such that a ij 0. Let y be the 1 m matrix such that the jth column is 1 and 0 elsewhere. Then the (1, j th entry of ya is a ij 0, a contradiction

27 Exercise 118 An n n matrix A is said to be idempotent if A 2 = A. (a Show that the matrix A = 1 ( is idempotent (b Show that if A is idempotent then the matrix (I n A is also idempotent. (a Easy calculation shows that A 2 = A. (b Suppose that A 2 = A then (I n A 2 = I n 2A+A 2 = I n 2A+A = I n A 27 Exercise 119 The purpose of this exercise is to show that the rule (ab n = a n b n does not hold with matrix multiplication. Consider the matrices A = Show that (AB 2 A 2 B 2. ( , B = ( We have and (AB 2 = A 2 B 2 = ( ( Exercise 120 Show that AB = BA if and only if A T B T = B T A T. AB = BA if and only if (AB T = (BA T if and only if B T A T = A T B T Exercise 121 Suppose AB = BA and n is a non-negative integer. (a Use induction to show that AB n = B n A. (b Use induction and (a to show that (AB n = A n B n.

28 28 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS (a The equality holds for n = 0 and n = 1. Suppose that AB n = B n A. Then AB n+1 = (ABB n = (BAB n = B(AB n = B(B n A = B n+1 A. (b The equality holds for n = 1. Suppose that (AB n = A n B n. Then (AB n+1 = (AB n (AB = (A n B n (AB = (A n B n (BA = A n (B n BA = A n (B n+1 A = A n (AB n+1 = A n+1 B n+1 Exercise 122 Let A and B be symmetric matrices. Show that AB is symmetric if and only if AB = BA. AB is symmetric if and only if (AB T = AB if and only if B T A T = AB if and only if AB = BA Exercise 123 Show that tr(aa T is the sum of the squares of all the entries of A. If A = (a ij then A T = (a ji. Hence, AA T = ( n k=1 a ika jk. Using the definition of the trace we have tr(aa T = n n ( a ik a ik = i=1 Exercise 124 (a Find two 2 2 singular matrices whose sum in nonsingular. (b Find two 2 2 nonsingular matrices whose sum is singular. k=1 n i=1 n k=1 a 2 ik (a (b A = A = ( ( ( 0 0, B = 0 1 ( 1 0, B = 0 1

29 29 Exercise 125 Show that the matrix is singular. A = If B is a 3 3 matrix such that BA = I 3 then b 31 (0 + b 32 (0 + b 33 (0 = 0 But this is equal to the (3, 3 entry of I 3 which is 1. A contradiction Exercise 126 Let Find A 3. A = ( A 3 = (A 1 3 = ( = ( Exercise 127 Let Find A. A 1 = ( Using Exercise??, we find A = (A 1 1 = ( Exercise 128 Let A and B be square matrices such that AB = 0. Show that if A is invertible then B is the zero matrix

30 30 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS If A is invertible then B = I n B = (A 1 AB = A 1 (AB = A 1 0 = 0 Exercise 129 Find the inverse of the matrix ( sin θ cos θ A = cos θ sin θ Using Exercise?? we find ( sin θ cos θ A 1 = cos θ sin θ Exercise 130 Which of the following are elementary matrices? (a (b ( (c (d (a No. This matrix is obtained by performing two operations: r 2 r 3 and r 1 r 1 + r 3. (b Yes: r 2 r 2 5r 1. (c Yes: r 2 r 2 + 9r 3. (d No: r 1 2r 1 and r 1 r 1 + 2r 4

31 Exercise 131 Let A be a 4 3 matrix. Find the elementary matrix E, which as a premultiplier of A, that is, as EA, performs the following elementary row operations on A : (a Multiplies the second row of A by -2. (b Adds 3 times the third row of A to the fourth row of A. (c Interchanges the first and third rows of A. 31 (a (b (c Exercise 132 For each of the following elementary matrices, describe the corresponding elementary row operation and write the inverse. (a E = (b E =

32 32 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS (c (a r 1 r 3, E 1 = E. (b r 2 r 2 2r 1 (c r 3 5r 3 E = Exercise 133 Consider the matrices A = 2 7 1, B = , C = Find elementary matrices E 1, E 2, E 3, and E 4 such that (ae 1 A = B, (be 2 B = A, (ce 3 A = C, (de 4 C = A (a (b E 2 = E 1. (c E 1 = E 3 =

33 33 (d E 4 = Exercise 134 Determine if the following matrix is invertible Consider the matrix Step 1: r 2 r 2 2r 1 and r 3 r 3 + r Step 2: r 3 r 3 + r It follows that the given matrix is row equivalent to a matrix with a row of zeros. By Theorem?? this matrix is singular Exercise 135 For what values of a does the following homogeneous system have a nontrivial solution? { (a 1x1 + 2x 2 = 0 2x 1 + (a 1x 2 = 0

34 34 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Let A be the coefficient matrix of the given system, i.e. ( a 1 2 A = 2 a 1 By Exercise??, A is invertible if and only if (a ,i.e. a 3 and a 1. In this case the trivial solution is the only solution. Hence, in order to have nontrivial solution we must have a = 1 or a = 3 Exercise 136 Find the inverse of the matrix We first construct the matrix Step 1: r 3 r 3 5r Step 2: r 2 1r 2 2 and r 3 1r Step 3: r 1 r 1 r

35 35 Step 4: r 2 r 2 3r 2 3 and r 1 r 1 + 1r It follows that A 1 = Exercise 137 What conditions must b 1, b 2, b 3 satisfy in order for the following system to be consistent? x 1 + x 2 + 2x 3 = b 1 x 1 + x 3 = b 2 2x 1 + x 2 + 3x 3 = b 3 The augmented matrix of the system is b b b 3 Step 1: r 2 r 2 r 1 and r 3 r 3 2r b b 2 b b 3 2b 1 Step 2: r 3 r 3 r b b 2 b b 3 b 2 b 1 The system is consistent if and only if b 3 b 2 b 1 = 0 Exercise 138 Prove that if A is symmetric and nonsingular than A 1 is symmetric.

36 36 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Let A be an invertible and symmetric n n matrix. Then (A 1 T = (A T 1 = A 1. That is, A 1 is symmetric Exercise 139 If find D 1. D = According to Exercise?? we have D 1 = Exercise 140 Prove that a square matrix A is nonsingular if and only if A is a product of elementary matrices. Suppose first that A is nonsingular. Then by Theorem??, A is row equivalent to I n. That is, there exist elementary matrices E 1, E 2,, E k such that I n = E k E k 1 E 1 A. Then A = E1 1 E2 1 E 1 k. But each E 1 i is an elementary matrix by Theorem??. Conversely, suppose that A = E 1 E 2 E k. Then (E 1 E 2 E k 1 A = I n. That is, A is nonsingular Exercise 141 Prove that two m n matrices A and B are row equivalent if and only if there exists a nonsingular matrix P such that B = P A. Suppose that A B. Then there exist elementary matrices E 1, E 2,, E k such that B = E k E k 1 E 1 A. Let P = E k E k 1 E 1. Then by Theorem?? and Theorem?? (a, P is nonsingular. Conversely, suppose that B = P A, for some nonsingular matrix P. By Theorem??, P I n. That is, I n = E k E k 1 E 1 P. Thus, B = E1 1 E2 1 E 1 k A and this implies that A B

37 Exercise 142 Let A and B be two n n matrices. Suppose A is row equivalent to B. Prove that A is nonsingular if and only if B is nonsingular. Suppose that A B. Then by the previous exercise, B = P A, with P nonsingular. If A is nonsingular then by Theorem?? (a, B is nonsingular. Conversely, if B is nonsingular then A = P 1 B is nonsingular Exercise 143 Show that the product of two lower (resp. upper triangular matrices is again lower (resp. upper triangular. We prove by induction on n that the product of tow lower triangular matrices is lower triangular. For n = 2 we have ( ( a11 0 b11 0 AB = ( a 21 a 22 b 21 b 22 a = 11 b 11 0 a 21 b 11 + a 22 b 21 a 22 b 22 Thus, AB is lower triangular. So suppose that the result is true for all lower triangular matrices of sise n 1. Let A and B be two lower triangular matrices of size n n. Then ( ( a 0 b 0 AB = = ( X A 1 Y B 2 ab 0 bx + A 1 Y A 1 B 1 where X and Y are (n 1 1 matrices and A 1, B 1 are (n 1 (n 1 lower triangular matrices. By the induction hypothesis, A 1 B 1 is lower triangular so that AB is lower triangular. The fact that the product of two upper triangular matrices is upper triangular follows by transposition. Exercise 144 Show that a 2 2 lower triangular matrix is invertible if and only if a 11 a 22 0 and in this case the inverse is also lower triangular. 37

38 38 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS By Exercise??, the lower triangular matrix ( a11 0 A = a 21 a 22 is invertible if and only in a 11 a 22 0 and in this case A 1 = ( 1 a 11 0 a 21 1 a 11 a 22 a 22 Exercise 145 Let A be an n n matrix and suppose that the system Ax = 0 has only the trivial solution. Show that A k x = 0 has only the trivial solution for any positive integer k. Since Ax = 0 has only the trivial solution then A is invertible. By indiction on k and Theorem??(a, A k is invertible and consequently the system A k x = 0 has only the trivial solution Exercise 146 Show that if A and B are two n n matrices then A B. Since A is invertible then A I n. That is, there exist elementary matrices E 1, E 2,, E k such that I n = E k E k 1 E 1 A. Similarly, there exist elementary matrices F 1, F 2,, F l such that I n = F l F l 1 F 1 B. Hence, A = E1 1 E2 1 E 1 F lf l 1 F 1 B. That is, A B k Exercise 147 Show that an n n matrix invertible matrix A satisfies the property (P: If AB = AC then B = C. Suppose that A is invertible. Let B abd C be two n n matrices such that AB = AC. Then A(B C = 0. Multiplying both sides by A 1 to obtain B C = 0 or B = C

39 Exercise 148 Show that an n n matrix A is invertible if and only if the equation ya = 0 has only the trivial solution. Suppose that A is invertible and let y be a 1 n matrix such that ya = 0. Multiplying from the left by A 1 we find y = 0. Conversely, suppose that the equation ya = 0 has only the trivial solution. Then A T x = 0 has only the trivial solution. By Theorem??, A T is invertible and consequently A is invertible Exercise 149 Let A be an n n matrix such that A n = 0. Show that I n A is invertible and find its inverse. We have (I n A(I n +A+A 2 + +A n 1 = I n +A+ +A n 1 A A 2 A n 1 A n = I n. Thus, I n A is invertible and (I n A 1 = I n +A+ +A n 1 39 Exercise 150 Let A = (a ij (t be an m n matrix whose entries are differentiable functions of the variable t. We define da = ( da ij. Show that if the entries in A and dt dt B are differentiable functions of t and the sizes of the matrices are such that the stated operations can be performed, then (a d da (ka = k. dt dt (b d da (A + B = + db. dt dt dt (c d da (AB = B + A db. dt dt dt (a d (ka = ( d (ka dt dt ij = (k da ij = k( da ij = k da. dt dt dt (b d (A + B = ( d (a dt dt ij + b ij = ( da ij + db ij = da dt dt (c Use the product rule of differentiation + db. dt dt Exercise 151 Let A be an n n invertible matrix with entries being differentiable functions of t. Find a formula for da 1 dt.

40 40 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Since A is invertible then AA 1 = I n. Taking derivative of both sides and using part (c of the previous exercise we find da dt A 1 + A da 1 = 0. Thus, dt A da 1 = da dt dt A 1. Now premultiply by A 1 to obtain da 1 1 da = A dt dt A 1 Exercise 152 (Sherman-Morrison formula Let A be an n n invertible matrix. Let u, v be n 1 matrices such that v T A 1 u Show that (a (A + uv T (A 1 A 1 uv T A 1 = I 1+v T A 1 u n. (b Deduce from (a that A is invertible. (a Using the properties of matrix multiplication we find (A + uv T (A 1 A 1 uv T A 1 = I 1+v T A 1 u n uvt A v T A 1 u uvt A 1 uvt A 1 uv T A 1 = 1+v T A 1 u I n + uvt A 1 v T A 1 u uv T A 1 uv T A 1 = I 1+v T A 1 u n + vt A 1 u(uv T A 1 uv T A 1 = I 1+v T A 1 u n. (b It follows from Theorem?? and (athat A + uv T is invertible and (A + uv T 1 = A 1 A 1 uv T A v T A 1 u Exercise 153 Let x = (x 1, x 2, x m T be an m 1 matrix and y = (y 1, y 2,, y n T be an n n matrix. Construct the m n matrix xy T. Using matrix multiplication we have xy T = = x 1 x 2. x m (y 1, y 2,, y n x 1 y 1 x 1 y 2 x 1 y n x 2 y 1 x 2 y 2 x 2 y n... x m y 1 x m y 2 x m y n

41 Exercise 154 Show that a triangular matrix A with the property A T = A 1 is always diagonal. Since A is invertible and upper triangular then A 1 is also upper triangular. Since A is upper triangular then A T is lower triangular. But A T = A 1. Thus, A T is both upper and lower triangular so it must be diagonal. Hence, A is also diagonal Chapter 3 Exercise 186 (a Find the number of inversions in the permutation ( (b Is this permutation even or odd? 41 (a There are five inversions: (41, (43, (42, (32, (52. (b The given permutation is odd Exercise 187 Evaluate the determinant of each of the following matrices (a ( 3 5 A = 2 4 (b A = (a A = (3(4 ( 2(5 = 22. (b A = 2( ( (40 3 = 0 Exercise 188 Find all values of t for which the determinant of the following matrix is zero. t A = 0 t t 1

42 42 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS A = t(t 1(t 4 = 0 implies t = 0, t = 1, or t = 4 Exercise 189 Solve for x x x = x x 5 Evaluating the determinants on both sides we find x(1 x + 3 = x(x (6 x Simplifying to obtain the quadratic equation 2x 2 3x 3 = 0 whose roots are x 1 = 3 33 and x 4 2 = Exercise 190 Evaluate the determinant of the following matrix A = Since the given matrix has a row consisting of 0 then A = 0 Exercise 191 Evaluate the determinant of the following matrix The given matrix is upper triangular so that the determinant is the product of entries on the main diagonal, i.e. equals to 1296

43 Exercise 192 Use the row reduction technique to find the determinant of the following matrix. A = Step 1: r 1 r 1 + r 4 B 1 = and B 1 = A. Step 2: r 2 r 2 + 2r 1 and r 3 r 3 + r 4 B 2 = and B 2 = A. Step 3: r 4 r 4 + r 1 and B 3 = A. Step 4: r 2 r 2 2r 3 B 3 = B 4 =

44 44 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS and B 4 = A. Step 5: r 3 r 3 + 3r 2 and r 4 r 4 + 7r 2 B 5 = and B 5 = A. Step 6: r r 3 and B 6 = 1 2 A. Step 7: r 4 r 4 5r 3 B 6 = B 7 = and B 7 = 1 2 A. Thus, A = 2 B 7 = 4 Exercise 193 Given that find (a (b a b c d e f g h i d e f g h i a b c = 6,, 3a 3b 3c d e f 4g 4h 4i

45 45 (c (d a + g b + h c + i d e f g h i 3a 3b 3c d e f g 4d h 4e i 4f (a The process involves row interchanges twice so d e f g h i a b c = 6 (b We have a b c d e f g h i = 1 3 3a 3b 3c d e f g h i = a 3b 3c d e f 4g 4h 4i = a 3b 3c d e f 4g 4h 4i Thus, (c (d a b c d e f g h i = 1 3 3a 3b 3c d e f 4g 4h 4i a + g b + h c + i d e f g h i 3a 3b 3c d e f g h i = = 72 a b c d e f g h i = 1 3 = 6 3a 3b 3c d e f g 4d h 4e i 4f

46 46 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Thus, 3a 3b 3c d e f g 4d h 4e i 4f = 18 Exercise 194 Determine by inspection the determinant of the following matrix The determinant is 0 since the first and the fifth rows are proportional Exercise 195 Find the determinant of the 1 1 matrix A = (3. By definition the determinant of a 1 1 matrix is just the entry of that matrix. Thus, A = 3 Exercise 196 Let A be a 3 3 matrix such that 2A = 6. Find A. We have, 6 = 2A = 2 3 A = 8 A. Thus, A = 3 4 Exercise 197 Show that if n is any positive integer then A n = A n. The proof is by induction on n 1. The equality is valid for n = 1. Suppose that it is valid up to n. Then A n+1 = A n A = A n A = A n A = A n+1 Exercise 198 Show that if A is an n n skew-symmetric and n is odd then A = 0.

47 Since A is skew-symmetric then A T = A. Taking the determinant of both sides we find A T = A = ( 1 n A = A since n is odd. Thus, 2 A = 0 and therefore A = 0 Exercise 199 Show that if A is orthogonal, i.e. A T A = AA T = I n then A = ±1. Note that A 1 = A T. Taking to determinant of both sides of the equality A T A = I n to obtain A T A = 1 or A 2 = 1 since A T = A. It follows that A = ±1 Exercise 200 If A is a nonsingular matrix such that A 2 = A, what is A? Taking the determinant of both sides to obtain A 2 = A or A ( A 1 = 0. Hence, either A is singular or A = 1 Exercise 201 True or false: If A = then rank(a = 3. Justify your answer Since A has a column of zero then A = A T = 0. In this case, rank(a < 3 47 Exercise 202 Find out, without solving the system, whether the following system has a nontrivial solution x 1 2x 2 + x 3 = 0 2x 1 + 3x 2 + x 3 = 0 3x 1 + x 2 + 2x 3 = 0

48 48 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS The coefficient matrix A = has determinant A = 0. By Theorem??, the system has a nontrivial solution Exercise 203 For which values of c does the matrix 1 0 c A = c 4 have an inverse. Finding the determinant we get A = 2(c + 2(c 3. The determinant is 0 if c = 2 or c = 3 Exercise 204 If A = 2 and B = 5, calculate A 3 B 1 A T B 2. A 3 B 1 A T B 2 = A 3 B 1 A B 2 = A 4 B = 80 Exercise 205 Show that AB = BA. We have AB = A B = B A = BA Exercise 206 Show that A + B T = A T + B for any n n matrices A and B. We have A + B T = (A + B T T = A T + B

49 Exercise 207 Let A = (a ij be a triangular matrix. Show that A 0 if and only if a ii 0, for 1 i n. Let A = (a ij be a triangular matrix. By Theorem??, A is nonsingular if and only if A 0 and this is equivalent to a 11 a 22 a nn 0 Exercise 208 Express a 1 + b 1 c 1 + d 1 a 2 + b 2 c 2 + d 2 as a sum of four determinants whose entries contain no sums. 49 a 1 + b 1 c 1 + d 1 a 2 + b 2 c 2 + d 2 = a 1 c 1 a 2 c 2 = a 1 d 1 a 2 d 2 = b 1 c 1 b 2 c 2 = b 1 d 1 b 2 d 2 Exercise 209 Let (a Find adj(a. (b Compute A. A = (a The matrix of cofactors is C 11 C 12 C 13 C 21 C 22 C 23 C 31 C 32 C 33 = The adjoint of A is the transpose of the cofactors matrix adj(a = (b A = 18( ( ( = 94

50 50 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 210 Find the determinant of the matrix A = Expanding along the first row A = 3C C C C 14 = 15 Exercise 211 Find the determinant of the following Vandermonde matrix A = a b c a 2 b 2 c 2 We have a b c a 2 b 2 c 2 = a b c c a 2 b 2 c 2 c 2 = = a b c c a a 2 b 2 c 2 c 2 a 2 b c c a b 2 c 2 c 2 a 2 = (b c(c 2 a 2 (b 2 c 2 (c a = (b c(c a(a b Exercise 212 Let A be an n n matrix. Show that adj(a = A n 1. Suppose first that A is invertible. Then adj(a = A 1 A so that adj(a = A A 1 = A n A 1 = A n = A n 1. If A is singular then adj(a is singular. A

52 52 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS (a A = 1( ( (10 3 = 2. (b The matrix of cofactors of A is The adjoint is the transpose of this cofactors matrix adj(a (c A 1 = adj(a A = Exercise 217 Prove that if A is symmetric then adj(a is also symmetric. Suppose that A T = A. Then (adj(a T = ( A A 1 T = A (A 1 T = A (A T 1 = A A 1 = adj(a Exercise 218 Prove that if A is a nonsingular triangular matrix then A 1 is also triangular. Suppose that A = (a ij is a lower triangular invertible matrix. Then a ij = 0 if i < j. Thus, C ij = 0 if i > j since in this case C ij is the determinant of a lower triangular matrix with at least one zero on the diagonal. Hence, adj(a is lower triangular and A 1 = adj(a is also lower triangular A Exercise 219 Let A be an n n matrix. (a Show that if A has integer entries and A = 1 then A 1 has integer entries as well. (b Let Ax = b. Show that if the entries of A and b are integers and A = 1 then the entries of x are also integers.

53 (a If A has integer entries then adj(a has integer entries. If A = 1 then A 1 = adj(a has integer entries. (b Since A = 1 then A is invertible and x = A 1 b. By (a, A 1 has integer entries. Since b has integer entries then A 1 b has integer entries Exercise 220 Show that if A k = 0 for some positive integer k then A is singular. Suppose the contrary. Then we can multiply the equation A k = 0 by A 1 k 1 times to obtain A = 0 which is a contradiction Exercise 221 Use Cramer s Rule to solve x 1 + 2x 3 = 6 3x 1 + 4x 2 + 6x 3 = 30 x 1 2x 2 + 3x 3 = 8 53 A = A 1 = A 2 = A 3 = , A = 44., A 1 = 40., A 2 = 72., A = 152. Thus, x 1 = A 1 A = 10 11, x 2 = A 2 A = 18 11, x 3 = A 3 A = 38 11

54 54 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 222 Use Cramer s Rule to solve A = 5x 1 + x 2 x 3 = 4 9x 1 + x 2 x 3 = 1 x 1 x 2 + 5x 3 = 2 A 1 = A 2 = A 3 = , A = 16., A 1 = 12., A 2 = 166., A 3 = 42. Thus, x 1 = A 1 A = 3 4, x 2 = A 2 A = 83 8, x 3 = A 3 A = 21 8 Chapter 4 Exercise 283 Show that the midpoint of P (x 1, y 1, z 1 and Q(x 1, y 2, z 2 is the point M( x 1 + x 2 2, y 1 + y 2 2, z 1 + z 2. 2 Let M = (x, y, z be the midpoint of the line segment P Q. Then OM = OP + P M = OP + 1 P Q 2 = (x 1, y 1, z 1 + 1(x 2 2 x 1, y 2 y 1, z 2 z 1 = ( x 1+x 2, y 1+y 2, z 1+z

55 Exercise 284 (a Let n = (a, b, c be a vector orthogonal to a plane P. Suppose P 0 (x 0, y 0, z 0 is a point in the plane. Write the equation of the plane. (b Find an equation of the plane passing through the point (3, 1, 7 and perpendicular to the vector n = (4, 2, 5. (a Let P = (x, y, z be an arbitrary point in the plane. Then the vector P 0 P = (x x 0, y y 0, z z 0 is contained in the plane. Since n is orthogonal to the plane then < n, P 0 P >= 0 and this leads to the equation a(x x 0 + b(y y 0 + c(z z 0 = 0. (b Here we have a = 4, b = 2, c = 5, x 0 = 3, y 0 = 1, and z 0 = 7. Substituting in the equation obtained in (a to find 4x + 2y 5z + 25 = 0 Exercise 285 Find the equation of the plane through the points P 1 (1, 2, 1, P 2 (2, 3, 1 and P 3 (3, 1, 2. First we find the normal vector as follows n = P 1 P 2 i j k P 1 P 3 = = 9 i + j 5 k Using Exercise282 (a we find 9(x 1+(y 2 5(z 1 = 0 or 9x+y 5z = 16 Exercise 286 Find the parametric equations of the line passing through a point P 0 (x 0, y 0, z 0 and parallel to a vector v = (a, b, c. Let P = (x, y, z be an arbitrary point on the line. Then P P 0 = t v; that is, (x x 0, y y 0, z z 0 = (ta, tb, tc. Hence, the parametric equations of the line are x = x 0 + ta, y = y 0 + bt, z = z 0 + tc Exercise 287 Compute < u, v > when u = (2, 1, 3 and v = (1, 4, 1. 55

56 56 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS < u, v >= (2(1 + ( 1(4 + (3( 1 = 5 Exercise 288 Compute the angle between the vectors u = ( 1, 1, 2 and v = (2, 1, 1. If θ denotes the angle between the vectors then Since 0 θ π then θ = 2π 3 cos θ = Exercise 289 For any vectors u and v we have < u, v > u v = 1 2. u v 2 = u 2 v 2 < u, v > 2. Suppose that u = (x 1, y 1, z 1 and v = (x 2, y 2, z 2. Then u v = (y 1 z 2 y 2 z 1, x 2 z 1 x 1 z 2, x 1 y 2 x 2 y 1. Thus, u v 2 = (y 1 z 2 y 2 z (x 2 z 1 x 1 z (x 1 y 2 x 2 y 1 2 On the other hand, u 2 v 2 < u, v > 2 = (x y z 2 1(x y z 2 2 (x 1 x 2 + y 1 y 2 + z 1 z 2 2. Expanding one finds that the given equality holds Exercise 290 Show that u v is the area of the parallelogram with sides u and v. Let θ be the angle between the vectors u and v. Constructing the parallelogram with sides u and v one finds that its area is u v sin θ. By the previous exercise, we have u v 2 = u 2 v 2 < u, v > 2 = u 2 v 2 u 2 v 2 sin 2 θ = u 2 v 2 (1 sin 2 θ = u 2 v 2 cos 2 θ Since θ [0, π] then sin θ 0 and therefore u v = u v sin θ

57 Exercise 291 Let P be the collection of polynomials in the indeterminate x. Let p(x = a 0 + a x + a 2 x 2 + and q(x = b 0 + b 1 x + b 2 x 2 + c... be two polynomials in P. Define the operations: (a Addition: p(x + q(x = a 0 + b 0 + (a 1 + b 1 x + (a 2 + b 2 x 2 + (b Multiplication by a scalar: αp(x = αa 0 + (αa 1 x + (αa 2 x 2 +. Show that P is a vector space. Since P is a subset of the vector space of all functions defined on R then it suffices to show that P is a subspace. Indeed, the sum of two polynomilas is again a plynomial and the scalar multiplication by a polynomial is also a plynomial Exercise 292 Define on R 2 the following operations: (i (x, y + (x, y = (x + x, y + y, ; (ii α(x, y = (αy, αx. Show that R 2 with the above operations is not a vector space. Let x y. Then α(β(x, y = α(βy, βx = (αβx, αβy (αβ(x, y then R 2 with the above operations is not a vector space Exercise 293 Let U = {p(x P : p(3 = 0}. Show that U is a subspace of P. Let p, q U and α R. Then αp + q is a polynomial such that (αp + q(3 = αp(3 + q(3 = 0. That is, αp + q U. This says that U is a subspace of P Exercise 294 Let P n denote the collection of all polynomials of degree n. Show that P n is a subspace of P. Let p(x = a 0 + a 1 x + + a n x n, q(x = b 0 + b 1 x + + b n x n, and α R. Then (αp + q(x(αa 0 b 0 + (αa 1 b 1 x + + (αa n b n x n P n. Thus, P n is a subspace of P 57

58 58 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS Exercise 295 Show that < f, g >= b f(xg(xdx is an inner product on the space C([a, b] a of continuous functions. (a < f, f >= b f 2 (xdx 0. If f 0 then < f, f >= 0. Conversely, if a < f, f >= 0 then by the continuity of f, we must have f 0. (b α < f, g >= α b f(xg(xdx = b (αf(xg(xdx =< αf, g >. a a (c < f, g >=< g, f >. (d < f, g + h >=< f, g > + < f, h > Exercise 296 Show that if u and v are two orthogonal vectors of an inner product space, i.e. < u, v >= 0, then u + v 2 = u 2 + v 2. Indeed, u + v 2 =< u + v, u + v >=< u, u > 2 < u, v > + < v, v >= u 2 + v 2 since < u, v >= 0 by orthogonality Exercise 297 (a Prove that a line through the origin in R 3 is a subspace of R 3 under the standard operations. (b Prove that a line not through the origin in R 3 in not a subspace of R 3. (a Let (L be a line through the origin with direction vector (a, b, c. Then the parametric equations of (L are given by x(t = ta, y(t = tb, z(t = tc. Let (x 1, y 1, z 1, (x 2, y 2, z 2 (L and α R. Then αx 1 + x 2 = (αt 1 + t 2 a = t 3 a where t 3 = αt 1 + t 2. Similarly, αy 1 + y 2 = t 3 b and αz 1 + z 2 = t 3 c. This shows that α(x 1, y 1, z 1 + (x 2, y 2, z 2 (L. (b Let (L be the line through the point ( 1, 4, 2 and with direction vector (1, 2, 3. Then the parametric equations of (L are given by x(t = 1 + t, y(t = 4 + 2t, z(t = 2 + 3t. It is clear that ( 1, 4, 2 (L, (0, 6, 5 (L but ( 1, 4, 2 + (0, 6, 5 = ( 1, 11, 7 (L

59 Exercise 298 Show that the set S = {(x, y : x 0} is not a vector space of R 2 under the usual operations of R 2. ( 1, 0 S but 2( 1, 0 = (2, 0 S so S is not a vector space Exercise 299 Show that the collection C([a, b] of all continuous functions on [a, b] with the operations: (f + g(x = f(x + g(x (αf(x = αf(x is a vector space. Since for any continuous functions f and g and any scalar α the function αf + g is continuous then C([a, b] is a subspace of F ([a, b] and hence a vector space Exercise 300 Let S = {(a, b, a + b : a, b R}. Show that S is a subspace of R 3 under the usual operations. Indeed, α(a, b, a+b+(a, b, a +b = (α(a+a, α(b+b, α(a+b+a +b S 59 Exercise 301 Let V be a vector space. Show that if u, v, w V are such that u + v = u + w then v = w. Using the properties of vector spaces we have v = v + 0 = v + (u + ( u = (v + u + ( u = (w + u + ( u = w + (u + ( u = w + 0 = w Exercise 302 Let H and K be subspaces of a vector space V. (a The intersection of H and K, denoted by H K, is the subset of V that consists of elements that belong to both H and K. Show that H V is a

60 60 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS subspace of V. (b The union of H and K, denoted by H K, is the susbet of V that consists of all elements that belong to either H or K. Give, an example of two subspaces of V such that H K is not a subspace. (c Show that if H K or K H then H K is a subspace of V. (a Let u, v H K and α R. Then u, v H and u, v K. Since H and K are subspaces then αu + v H and αu + v K that is αu + v H K. This shows that H K is a subspace. (b One can easily check that H = {(x, 0 : x R} and K = {(0, y : y R} are subspaces of R 2. The vector (1, 0 belongs to H and the vector (0, 1 belongs to K. But (1, 0 + (0, 1 = (1, 1 H K. It follows that H K is not a subspace of R 2. (c If H K then H K = K, a subspace of V. Similarly, if K H then H K = H, again a subspace of V Exercise 303 Let u and v be vectors in an inner product vector space. Show that Recall that u = < u, u >. Then u + v 2 + u v 2 = 2( u 2 + v 2. u + v 2 + u v 2 = < u + v, u + v > + < u v, u v > = < u, u > +2 < u, v > + < v, v > + < u, u > 2 < u, v > + < v, v > = 2(< u, u > + < v, v > = 2( u 2 + v 2 Exercise 304 Let u and v be vectors in an inner product vector space. Show that Indeed, u + v 2 u v 2 = 4 < u, v >. u + v 2 u v 2 = < u + v, u + v > < u v, u v > = < u, u > +2 < u, v > + < v, v > < u, u > +2 < u, v > < v, v > = 4 < u, v >

61 61 Exercise 305 (a Use Cauchy -Schwarz s inequality to show that if u = (x 1, x 2,, x n R n and v = (y 1, y 2,, y n R n then (x 1 y 1 + x 2 y x n y n 2 (x x x 2 n(y y y 2 n. (b Use (a to show n 2 (a 1 + a a n ( 1 a a a n where a i > 0 for 1 i n. (a The standard inner product in R n is given by < u, v >= x 1 y 1 +x 2 y x n y n. By Cauchy - Schwarz inequality we have that < u, v > 2 < u, u >< v, v > that is (x 1 y 1 + x 2 y x n y n 2 (x x x 2 n(y y y 2 n. (a Let x i = a i and y i = 1 ai in (a Exercise 306 Let C([0, 1] be the vector space of continuous functions on [0, 1]. Define < f, g >= 1 0 f(xg(xdx. (a Show that <.,. > is an inner product on C([0, 1]. (b Show that [ 1 2 [ 1 f(xg(xdx] 0 This inequality is known as Holder s inequality. (c Show that 0 ] [ 1 ] f(x 2 dx g(x 2 dx. 0 [ 1 (f(x + g(x 2 dx 0 ] 1 2 [ 1 0 ] 1 [ f(x ] 1 dx + g(x 2 2 dx. 0 This inequality is known as Minkowski s inequality.

62 62 CHAPTER 1. SOLUTIONS TO REVIEW PROBLEMS (a See Exercise 295 (b This is just the Cauchy-Schwarz s inequality. (c The inequality is definitely valid if 1 (f(x + 0 g(x2 dx = 0. So we may assume that this integral is nonzero. In this case, we have (f(x + 0 g(x2 dx = (f(x + g(x(f(x + g(xdx = 1 f(x(f(x + g(xdx + 1 g(x(f(x + g(xdx 0 0 [ ] 1 [ 1 f 2 2 ] 1 1 (xdx (f(x g(x2 2 dx [ ] 1 [ g2 2 ] 1 1 (xdx (f(x + 0 g(x2 2 dx Now divide both sides of this inequality by the desired inequality [ ] 1 1 (f(x + 0 g(x2 2 dx to obtain Exercise 307 Let W = span{v 1, v 2,, v n }, where v 1, v 2,, v n are vectors in V. Show that any subspace U of V containing the vectors v 1, v 2,, v n must contain W, i.e. W U. That is, W is the smallest subspace of V containing v 1, v 2,, v n. Let U be a subspace of V containing the vectors v 1, v 2,, v n. Let x W. Then x = α 1 v 1 + α 2 v α n v n for some scalars α 1, α 2,, α n. Since U is a subspace then x U. This gives x U and consequently W U Exercise 308 Express the vector u = ( 9, 7, 15 as a linear combination of the vectors v 1 = (2, 1, 4, v 2 = (1, 1, 3, v 3 = (3, 2, 5. The equation u = α v 1 + β v 2 + γ v 3 gives the system 2α + β + 3γ = 9 α β + 2γ = 7 4α + 3β + 5γ = 15 Solving this system (details omitted we find α = 2, β = 1 and γ = 2

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