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Unformatted text preview: MA 35100 HOMEWORK ASSIGNMENT #4 SOLUTIONS Problem 1. pg. 77; prob. 6 If possible, compute the matrix products in Exercises 1 through 13, using pencil and paper. a b c d d b c a Solution: We use Theorem 2.3.4 on page 73 of the text. The product is given by a b c d d b c a = a d + b ( c ) a ( b ) + b a c d + d ( c ) c ( b ) + d a = ad bc ad bc Remark: Given a 2 2 matrix A , we denote the cofactor of A as the 2 2 matrix cof( A ) = d b c a where A = a b c d . The previous exercise shows that we have the product A cof( A ) = det( A ) I 2 in terms of the 2 2 identity matrix I 2 . Problem 2. pg. 77; prob. 7 If possible, compute the matrix products in Exercises 1 through 13, using pencil and paper. 1 1 1 1 1 1 2 1 2 3 3 2 1 2 1 3 Solution: Again, we use Theorem 2.3.4 on page 73 of the text: 1 1 1 1 1 1 2 1 2 3 3 2 1 2 1 3 = 1 1 + 0 3 + ( 1) 2 1 2 + 0 2 + ( 1) 1 1 3 + 0 1 + ( 1) 3 1 + 1 3 + 1 2 2 + 1 2 + 1 1 3 + 1 1 + 1 3 1 1 + ( 1) 3 + ( 2) 2 1 2 + ( 1) 2 + ( 2) 1 1 3 + ( 1) 1 + ( 2) 3 =  1 1 5 3 4 6 2 4 Problem 3. pg. 77; prob. 8 1 If possible, compute the matrix products in Exercises 1 through 13, using pencil and paper. 0 1 0 0 0 1 0 0 Solution: Once more we use Theorem 2.3.4 on page 73 of the text: 0 1 0 0 0 1 0 0 = 0 + 1 0 0 1 + 1 0 + 0 0 0 1 + 0 = 0 0 0 0 Problem 4. pg. 78; prob. 55 In Exercises 55 through 64, find all matrices X that satisfy the given matrix equation. 1 2 2 4 X = 0 0 0 0 . Solution: The product of an n p matrix by a p m matrix is an n m matrix. As the matrix on the left is a n p = 2 2 matrix and the matrix on the right is a n m = 2 2 matrix, we must have n = m = p = 2. We denote the 2 2 matrix X as X = a 11 a 12 a 21 a 22 so that 1 2 2 4 X = a 11 + 2 a 21 a 12 + 2 a 22 2 a 11 + 4 a 21 2 a 12 + 4 a 22 ....
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This note was uploaded on 10/18/2010 for the course MATH 351 taught by Professor Egoins during the Spring '10 term at Purdue UniversityWest Lafayette.
 Spring '10
 EGoins
 Linear Algebra, Algebra

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