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problem with imag command when evaluating symbolic expressions.

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Carlos
Carlos 2013-2-11
Hi, when I execute the following code in Matlab
syms x y; f=4*i+9*x+y; imag(f)
I get the following result
ans =
(9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4
and the result should be 4 instead of what I get. I cannot figure out where have I gone wrong.Any ideas?
Thank you in advance.

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采纳的回答

Walter Roberson
Walter Roberson 2013-2-11
The result is correct. The symbolic toolbox assumes that variables might be complex unless it has been told otherwise. Try
syms x y real

更多回答（1 个）

Carlos
Carlos 2013-2-11
The answer is right, if you try subs((9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4),{x,y},{0,0}) or subs((9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4),{x,y},{1,1}) or you give x and y any value, the answer is always 4. It was just that I found the answer a bit awkward. From your answer I grasp that the only problem here was that I should have declared x and y as symbolic real variables.
Than you very much Walter.

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