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June 24th, 2010, 04:23 PM   #1
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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
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June 24th, 2010, 05:21 PM   #2
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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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