June 24th, 2010, 04:23 PM  #1 
Newbie Joined: Jun 2010 Posts: 24 Thanks: 0  Dimensions
If I was given a 4x5 matrix B and a 5x4 matrix A, what are the dimensions of BA? Is it 5x5 or 4x4 or BA is not defined? thanks 
June 24th, 2010, 05:21 PM  #2 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Dimensions
Write the dimensions from left to right. i.e. 4x5 by 5x4. For the resulting matrix to be defined, the inner dimensions must be equal. 5=5... check. The outer dimensions (from left to right) are the resulting dimensions. 4x4. To generalize, a CxD matrix times a DxE matrix has dimensions CxE. 

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