I have defined the following function, which is the Euler relation for $\cos\,x$.
f[x_] := 1/2*(Exp[I*k*x] + Exp[-I*k*x ] )
I want to take the derivative with respect to
x, but I am not getting the "right" answer. How do I do this?
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I'm not sure what you're looking for in a 'right' answer, but here are some ways to take (and view) the derivative of your function: