Undefined function or variable f when using solve function

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jeremy meyer
jeremy meyer 2016-12-1
编辑： Stephen23 2016-12-1
solve(RA(12)==1/f*((2/(k+1))*(1+(k-1)/2*f^2))^((k+1)/(2*k-2)),f)
"Undefined function or variable 'f'."
I want matlab to solve for when the correct positive f value makes the equation true. Currently I get a message that reads, Undefined function or variable 'f'. All I can think is duh, that's what I want you to solve for!!! Can someone else tell me why matlab doesn't understand the command and how to fix it? Also, there will be a positive and negative root. Any suggestions on how to tell matlab to just give me the positive root? Thanks!
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Stephen23
Stephen23 2016-12-1
编辑：Stephen23 2016-12-1
@jeremy meyer: did you read the solve documentation? It clearly shows how to specify a variable as being symbolic, which is all you need to do.

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回答（2 个）

John D'Errico
John D'Errico 2016-12-1
Did you ever tell MATLAB in advance that f is a symbolic variable?
help syms
Walter Roberson
Walter Roberson 2016-12-1
Before that you need to
syms f
You can further restrict that as
syms f positive
but you should still cross-check the result as sometimes constraints get ignored.

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