Katherine P. answered 03/04/13
Positive, Effective, and Experienced Math Tutor
The formula you want to remember (or write in your notes) is:
( m x n ) x ( n x p ) = ( m x p )
The columns of the first matrix (n) need to equal the rows of the second matrix (n).
In your case, you have:
DC = (3 x 5) x ( 5 x 1)
Applying ( m x n ) x ( n x p ) = ( m x p ), you get the dimensions of DC:
( 3 x 1)
Hope that helps!