Question: if a and b are invertible matrices then (ab)-1 = a-1b-1?
Answer: The inverse of an invertible matrix is denoted A-1. Also, when a matrix is invertible, so is its inverse, and its inverse's inverse is itself, (A-1)-1 = A. ... If A and B are both invertible, then their product is, too, and (AB)-1 = B-1A-1.