I am wondering for the matrices.

If $(A+B)C(D+E)=ACD+ACE+BCD+BCE$ holds?

I think for $(A+B)C = AC+BC$, but how about the above?

closed as off-topic by amWhy, Frits Veerman, Arnaldo, kingW3, NCh Jun 22 '17 at 16:11

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up vote 3 down vote accepted

You are close, but instead of $ABC$ you should have $ACD$. Note that matrix multiplication is associative; combining that with distributivity as you mentioned gives the desired result.

Edit: Please see @Evargalo's important comment below which points out that one should be careful when treating matrices like numbers, as matrix multiplication is not commutative.

  • I mention it in a comment because it doesn't deserve a different answer, but the only case where you have to be careful before developing product of matrix "as if they were numbers" is when you have issue with commutativity: for instance $(A+B)(B+A)=A^2+AB+BA+B^2\neq A^2+2AB+B^2$ in general. I suppose it is that kind of issue that caused the OP's extreme prudency ! – Evargalo Jun 21 '17 at 16:26
  • 1
    @Evargalo this is a very important comment. Thank you for adding it – ThomasGrubb Jun 21 '17 at 16:27

$$(A+B)C(D+E)=(AC+BC)(D+E)=ACD+ACE+BCD+BCE$$

By the way, $(A+B)C=AC+BC$.

  • 1
    oh it was my typo. Thank you very much. – Woonghee Lee Jun 21 '17 at 16:25
  • No problem. Glad to help. – Alberto Andrenucci Jun 21 '17 at 16:27

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