Undefined function or variable f when using solve function

 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!

[Stephen23]

@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.

Answer 1

[John D'Errico]

Did you ever tell MATLAB in advance that f is a symbolic variable?

help syms

Answer 2

[Walter Roberson]

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.