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'subs' not working as i want

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Fernando Pérez Lara
评论： Fernando Pérez Lara 2018-12-9
Hello guys,
I have this equation -x^2/2 + 2*x - (5*I)/2 and I'm trying to replace the letter x with a the matrix A and the letter I with the matrix B.
A = 0 1 0
0 0 1
2 -5 4
B = eye(3)
The problem is that when I write subs(eq, x, A) and subs(eq, I, eye(3)) I get a very big matrix, and this is not the answer.
  2 个评论
YT
YT 2018-12-8
编辑：YT 2018-12-8
It makes sense that your result will be "a very big matrix". subs(eq, x, A) will give you a 3x3 matrix because you substitute x for a 3x3 matrix. With subs(eq, I, eye(3)) you are trying to substiute an identity matrix in your previous subsituted matrix. So what do you expect the result to be then?
Fernando Pérez Lara
Fernando Pérez Lara 2018-12-9
Yes, you are right, but I didn't know how to use the subs. One guy gave me the correct answer. Thank you for your contribution

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采纳的回答

Walter Roberson
Walter Roberson 2018-12-8
Do both subs at the same time
subs(eq, {x, I}, {A, eye(3)})

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