Math 240 Calculus III

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1 The Calculus III Summer 2013, Session II Tuesday, July 16, 2013

2 Agenda 1. of the determinant 2. determinants 3. of determinants

3 What is the determinant? Yesterday: Ax = b has a unique solution when A is square and nonsingular. Today: how to determine whether A is invertible. Example [ ] a b Recall that a 2 2 matrix is invertible as long as c d ad bc 0. The quantity ad bc is the determinant of this matrix and the matrix is invertible exactly when its determinant is nonzero.

4 What should the determinant be? We want to associate a number with a matrix that is zero if and only if the matrix is singular. An n n matrix is nonsingular if and only if its rank is n. For upper triangular matrices, the rank is the number of nonzero entries on the diagonal. To determine if every number in a set is nonzero, we can multiply them. The determinant of an upper triangular matrix, A = [a ij ], is the product of the elements a ii along its main diagonal. We write a 11 a 1n det(a) =.... = a 11 a 22 a nn. 0 a nn

5 What should the determinant be? What about matrices that are not upper triangular? We can make any matrix upper triangular via row reduction. So how do elementary row operations affect the determinant? M i (k) multiplies the determinant by k. (Remember that k cannot be zero.) A ij (k) does not change the determinant. P ij multiplies the determinant by 1. Let s extend these properties to all matrices. The determinant of a square matrix, A, is the determinant of any upper triangular matrix obtained from A by row reduction times 1 k for every M i(k) operation used while reducing as well as 1 for each P ij operation used.

6 determinants Example Compute det(a), where A = We need to put A in upper triangular form P A ( 2) M ( 5) A 23 ( 2) So the determinant is = ( 1)(5) = 15.

7 determinants Important Example [ ] a b Given a general 2 2 matrix, A =, compute det(a). c d [ ] [ ] a b A12( a) c a b c d 0 d bc a so a b c d = a b 0 d bc = ad bc. a This explains [ ] 1 a b = c d 1 ad bc [ ] d b when ad bc 0. c a

8 Other methods of computing determinants Theorem (Cofactor expansion) Suppose A = [a ij ] is an n n matrix. For any fixed k between 1 and n, n n det(a) = ( 1) k+j a kj det(a kj ) = ( 1) i+k a ik det(a ik ) j=1 i=1 where A ij is the (n 1) (n 1) submatrix obtained by removing the i th row and j th column from A. Example i j k a b c d e f = b e c f i a d c f j + a d b e k.

9 Other methods of computing determinants Corollary If A = [a ij ] is an n n matrix and the element a ij is the only nonzero entry in its row or column then det(a) = ( 1) i+j a ij A ij. Example = = 27.

10 The Other methods of computing determinants Some of you may have learned the method of computing a 3 3 determinant by multiplying diagonals. a 11 a 12 a 13 a 11 a 12 a 21 a 22 a 23 a 21 a 22 a 31 a 32 a 33 a 31 a Be aware that this method does not work for matrices larger than 3 3.

11 of determinants Theorem (Main theorem) Suppose A is a square matrix. The following are equivalent: A is invertible, det(a) 0. Further properties det ( A T ) = det(a). The determinant of a lower triangular matrix is also the product of the elements on the main diagonal. If A has a row or column of zeros then det(a) = 0. If two rows or columns of A are the same then det(a) = 0. det(ab) = det(a) det(b), det ( A 1) = det(a) 1. It is not true that det(a + B) = det(a) + det(b).

12 Geometric interpretation Let A be an n n matrix and a 1,..., a n be the rows or columns of A. Theorem The volume (or area, if n = 2) of the paralellepiped determined by the vectors a 1,..., a n is det(a). Source: en.wikibooks.org/wiki/linear Algebra Corollary The vectors a 1,..., a n lie in the same hyperplane if and only if det(a) = 0.

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