Math 215 HW #6 Solutions

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1 Math 5 HW #6 Solutions Problem 34 Show that x y is orthogonal to x + y if and only if x = y Proof First, suppose x y is orthogonal to x + y Then since x, y = y, x In other words, = x y, x + y = (x y) T (x + y) = x T x + x T y y T x y T x = x, x + x, y y, x y, y = x, x y, y = x y, so x = y Since the norm of a vector can never be negative, this implies that x = y Thus, we see that if x y is orthogonal to x + y, then x = y On the other hand, suppose x = y Then x y, x + y = (x y) T (x + y) = x T x + x T y y T x y T x = x, x + x, y y, x y, y = x, x y, y = x y =, so we see that x y is orthogonal to x + y We ve seen that the implication goes both ways, so we conclude that x y is orthogonal to x + y if and only if x = y, as desired Problem 3 Let S be a subspace of R n Explain what (S ) = S means and why it is true Answer: First, (S ) is the orthogonal complement of S, which is itself the orthogonal complement of S, so (S ) = S means that S is the orthogonal complement of its orthogonal complement To show that it is true, we want to show that S is contained in (S ) and, conversely, that (S ) is contained in S; if we can show both containments, then the only possible conclusion is that (S ) = S To show the first containment, suppose v S and w S Then v, w =

2 by the definition of S Thus, S is certainly contained in (S ) (which consists of all vectors in R n which are orthogonal to S ) To show the other containment, suppose v (S ) (meaning that v is orthogonal to all vectors in S ); then we want to show that v S I m sure there must be a better way to see this, but here s one that works Let {u,, u p } be a basis for S and let {w,, w q } be a basis for S If v / S, then {u,, u p, v} is a linearly independent set Since each vector in that set is orthogonal to all of S, the set {u,, u p, v, w,, w q } is linearly independent Since there are p+q+ vectors in this set, this means that p+q+ n or, equivalently, p + q n On the other hand, if A is the matrix whose ith row is u T i, then the row space of A is S and the nullspace of A is S Since S is p-dimensional, the rank of A is p, meaning that the dimension of nul(a) = S is q = n p Therefore, p + q = p + (n p) = n, contradicting the fact that p + q n v (S ), it must be the case that v S From this contradiction, then, we see that, if 3 Problem 38 This is a system of equations Ax = bs with no solution: x + y + z = 5 x + y + 3z = 5 3x + 4y + 5z = 9 Find numbers y, y, y 3 to multiply the equations so they add to = You have found a vector y in which subspace? The inner product is y T b is Answer: Let A = , x = x y z, and b = Then the given system is equivalent to the matrix equation Let y = Ax = b y y y 3 = Then multiplying the ith row in the system by y i and adding is equivalent to multiplying 5 5 9

3 both sides of the matrix equation by y T We see that x y T Ax = 3 y z On the other hand, = x + y + z x + y + 3z 3x + 4y + 5z = (x + y + z) + (x + y + 3z) (3x + 4y + 5z) = Therefore, y T Ax = y T b reduces to y T b = =, so we see that the system has no solution Since = y T Ax = y, Ax, = = we see that y is perpendicular to Ax no matter what x is Therefore, the vector y is perpendicular to the column space of A Since the orthogonal complement of col(a) is the left nullspace of A, we see that y must be an element of the left nullspace of A 4 Problem 336 Extend Problem 35 to a p-dimensional subspace V and a q-dimensional subspace W of R n What inequality on p + q guarantees that V intersects W in a nonzero vector? These subspaces cannot be orthogonal (Restatement: Suppose V is a p-dimensional subspace of R n and that W is a q-dimensional subspace of R n Moreover, suppose that V and W are orthogonal as subspaces of R n What must be true of the quantity p + q?) Answer: I claim that if p + q > n, then V and W intersect in some nonzero vector Equivalently, if V and W are orthogonal subspaces, then p + q n To see this, suppose p + q > n Let {v,, v p } be a basis for V and {w,, w q } be a basis for W Then, since R n is n-dimensional and since p + q > n, it must be the case that {v,, v p, w,, w q } is a linearly dependent set Thus, there exist real numbers c,, c p+q not all zero such that Stated another way, c v + + c p v p + c p+ w + + c p+q w q = c v + + c p v p = c p+ w c p+q w q The vector on the left-hand side is clearly an element of V, whereas the vector on the righthand side is just as clearly an element of W Therefore, the above vector (which is necessarily nonzero) is an element of the intersection V W 3

4 5 Problem 34 Suppose S is spanned by the vectors (,,, 3) and (, 3, 3, ) Find two vectors that span S This is the same as solving Axo for which A? Answer: Since the given vectors live in R 4 and are linearly independent, we see that S is -dimensional Thus, it makes sense that S should also be -dimensional Since we know that the orthogonal complement of the row space of any matrix is the nullspace of the matrix, we can determine S simply by constructing a matrix A whose row space is S and then determining the nullspace of A Clearly, if 3 A = 3 3 then the row space of A is equal to S To find the nullspace of A, we want to row reduce the augmented matrix First, subtract row from row : 3 Next, subtract twice row from row : 5 The corresponding system of equations is or, equivalently, x + 5x 4 = x + x 3 x 4 = x = 5x 4 x = x 3 + x 4 Hence, the solutions to the equation Ax = are of the form 5 x 3 + x 4 Thus,, is a basis for the nullspace of A, ie a basis for S 5 4

5 6 Problem 346 Find A T A if the columns of A are unit vectors, all mutually perpendicular Answer: Suppose A is m n, with columns a,, a n Then a T a T A T a T A = a a T a a T a n a a a n a T = a a T a a T a n a T n a T n a a T n a a T n a n In other words, the (i, j) entry of the matrix A T A is equal to a T i a j = a i, a j Since the columns are all mutually perpendicular, the above inner product will be zero unless i = j, meaning that the only nonzero entries in A T A will be on the diagonal Moreover, a i, a i = a i = since each column is a unit vector Therefore, A T A =, the identity matrix 7 Problem 38 The methane molecule CH 4 is arranged as if the carbon atom were at the center of a regular tetrahedron with four hydrogen atoms at the vertices If vertices are placed at (,, ), (,, ), (,, ), and (,, ) note that all six edges have length, so the tetrahedron is regular what is the cosine of the angle between the rays going from the center (,, ) to the vertices? (The bond angle itself is about 95, an old friend of chemists) Answer: θ In the above figure, the black dot is the carbon atom and the blue segments are the bonds between carbon and hydrogen atoms (the gray segments are the edges of the tetrahedron and 5

6 don t mean anything chemically) The goal is to determine the cosine of the angle θ between two of the blue segments The top left blue segment corresponds to the vector v = = whereas the top right blue segment corresponds to the vector w = = We know that so The numerator is v, w = v T w = v, w = v w cos θ, cos θ = v, w v w To find the first term in the denominator, note that so v = v, v = v T v = A similar calculation shows that as well, so Therefore, putting it all together, v = v = cos θ = w =, = = = 3 v w = 3 4 v, w v w = /4 3/4 = 3 = = 3 4, 6

7 8 Problem 34 What matrix P projects every point of R 3 onto the line of intersection of the planes x + y + t = and x t =? Answer: First, we need to find a vector on the line of intersection Note that any such vector is necessarily a solution of the matrix equation x y = t To solve this equation, we ll do Gaussian elimination on the augmented matrix Subtract row from row to get, Then add row to row and multiply row by : This corresponds to the system of equations or, equivalently, x x 3 = x + x 3 = x = x 3 x = x 3 Therefore, solutions to the matrix equation are of the form x 3, meaning that the vector a = is in the desired line of intersection Now, just recall that the projection matrix P is given by P = a at a T a 7

8 The denominator is simply a T a = = = 6 The numerator is Therefore, a a T = P = a at a T a = 6 = 4 =

9 9 Problem 38 Draw the projection of b onto a and also compute it from p = xa: cos θ (a) b = and a = (b) b = and a = sin θ Answer: (a) b p a Since x = at b, we just compute a T a a T cos θ b = sin θ = cos θ and so Therefore, (b) a T a = x = at b a T a = cos θ p = xa = cos θ = =, = cos θ cos θ b p a 9

10 Since x = at b, we just compute a T a and so Therefore, a T b = a T a = = = x = at b a T a = = p = xa = = + =, Problem 34 Project the vector b = (, ) onto the lines through a = (, ) and a = (, ) Draw the projections p and p and add p + p The projections do not add to b because the a s are not orthogonal Answer: = a p b p + p p = a First, notice that the projection of b onto the line through a is Since and we have that p = at b a T a a a T b = a T a = p = at b a T a a = = + = = + =, =

11 Next, the projection of b onto the line through a is p = at b a T a a Since and we have that Therefore, a T b = a T a = p = at b a T a a = 3 5 p + p = = + = 3 = + 4 = 5, 3/5 + 6/5 = = 3/5 6/5 8/5 6/5

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