Page 1 Math 141-copyright Joe Kahlig, 10B Section 2.5: Matrix Multiplication Multiplication of Matrices: Two matrices A and B can be multiplied such that AB = C provided that the dimension of A is m × n and the dimension of B is n × r. The resultant matrix C will have the dimension of m × r. Example: Give the dimension of the following products(if the multiplication). B C D A 3×4 3×7 4×3 4×4 AB AC CT D ACB Example: Find matrices A and B such that AB is possible and BA is not possible. Example: Compute " 1 4 2 5 3 6 # 7 −1 0 −2 1 −3 Page 2 Math 141-copyright Joe Kahlig, 10B Example: Compute 7 −1 0 −2 " 1 1 4 −3 2 5 3 6 # Definition: An identity matrix, denoted I, is a square matrix (n × n) that has I(1,1) = I(2,2) = ... = I(n,n) = 1 and all of the other entries equal to zero. The identity matrix has the property that IA = A and AI = A. (pick the size of the identity matrix so that the multiplication is possible. Example: Compute 1 0 0 " 1 4 2 5 3 6 # " 1 4 2 5 3 6 #" 1 0 " 1 4 2 5 #" 2 0 0 2 0 1 0 0 1 0 0 1 # # Example: Express the system of equations as a matrix equation. 3x + 2y = 7 x + 4y = 10 Page 3 Math 141-copyright Joe Kahlig, 10B Example: Matrix A shown the number of servings of each food that each person ate each day. Matrix B shows the number of units of fat, carbohydrates and protein in a serving of each food. A= Sandy David Bob Food A 3 0 2 Food B 1 5 2 Food C 2 6 0 B= Food A Food B Food C fat 25 31 30 carbs 8 24 12 Explain the meanings of the entries of BA = Food A Food B Food C = carbs 8 24 12 Sandy David Bob protein 12 19 22 89 189 134 Food A 3 0 2 Food B 1 5 2 72 192 64 99 131 134 = AB = fat 25 31 30 166 335 112 Sandy ∗ David Bob Food A 3 0 2 Food B 1 5 2 Food C 2 6 0 98 206 132 Food C 2 Food A ∗ 6 Food B 0 Food C 99 227 62 fat 25 31 30 carbs 8 24 12 protein 12 19 22 protein 12 19 22