Matrix Algebra
  H Matrix Operations

Glibert Strang (MIT)
Multiplication and Inverses of Matrices
(the first 20 minutes)

example 2

1, 2, 3

The number of columns of A must be the same as the number of rows in B

Problem 27-28
The Inverse of a Matrix

Proof that the Multiplication of the Elementary Matrices is the Inverse of A