In order for a matrix B to be the inverse of A, both equations AB D I and BA D I must be true. b. If A and B are n n and invertible, then A 1B 1 is the inverse of AB. c. If A D a b c d and ab cd ¤ 0; then A is invertible. d. If A is an invertible n n matrix, then the equation Ax D b is consistent for each b in Rn. e. Each elementary matrix is invertible.