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This question already has an answer here:

If

I1 = {{1,0},{1,-1}}

and

Acl = {{A , -B*K},{C*L, A-B*K-C*L}} 

where A, B, C, and D are not defined in the notebook. When I multiply the two matrices like this:

I1.Acl  

Mathematica gives me the following:

Not actually multiplied together

Whereas if I don't multiply the I1 and Acl but multiply the contents of the variables directly (with parenthesis around each element of the matrice). Mathematica gives me the answer as I would expect. Like this:

Desired form

How can I multiply a two symbolic matrices together such that I get an answer that an actual matrice multiplication instead of just displaying what I asked Mathematica to do. I'm sure this is a stupid question, but I searched Stack Exchange for 10 minutes using various search strings. In addition I searched Youtube, and Google.

Wolfram alpha gives me the answer as I would have expected. In the same form as if I had done the multiplication by hand. Mathematica won't.

Mathematica Version 11.3

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marked as duplicate by J. M. will be back soon Oct 1 '18 at 4:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Note also that you shouldn't use upper-case symbols in Mathematica. Built-in functions always begin with upper-case letters; and so user-defined functions and all symbolic variables should use lower-case letters. For example, in your case, C and K have built-in meaning, which could lead to unexpected results (you can tell whether a variable has been assigned a value/definition by the colouring). $\endgroup$ – AccidentalFourierTransform Oct 1 '18 at 1:26
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It looks your I1 matrix is being used while wrapped in a MatrixForm. Otherwise, it works just fine.

{{1, 0}, {1, -1}}.{{a, -b k}, {c l, a - b k - c l}}

{{a, -b k}, {a - c l, -a + c l}}

MatrixForm[{{1, 0}, {1, -1}}].{{a, -b k}, {c l, a - b k - c l}}

enter image description here

That being said, one could also have

MatrixForm[{{1, 0}, {1, -1}}][[1]].{{a, -b k}, {c l, a - b k - c l}}

{{a, -b k}, {a - c l, -a + c l}}

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  • $\begingroup$ Oh shoot. I did "wrap" it in MatrixForm. I thought MatrixForm was just a formatting command. I didn't think it did anything to my matrices. Thank you for your reply. That was it. $\endgroup$ – frequencydrive Sep 30 '18 at 23:23
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    $\begingroup$ @frequencydrive If you find the answer useful, please consider accepting it as an answer. $\endgroup$ – Johu Sep 30 '18 at 23:29
  • $\begingroup$ Related: mathematica.stackexchange.com/questions/166709 $\endgroup$ – John Doty Sep 30 '18 at 23:47

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