if a and b are invertible matrices of same order then is it necessary that inverse of ab exists?give examples

As A is a invertible matrix  so |A|  is not equal to 0
And B is a invertible matrix  so |B|  is not equal to 0
Hence |AB| is not equal to 0  or |AB| = |A||B|

Therefore AB is a invertible matrix , the necessary and sufficient condition is |AB| is not equal to zero.