# Taking derivative of Euler relation [closed]

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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## closed as too localized by Sjoerd C. de Vries, Szabolcs, m_goldberg, rm -rf♦May 12 '13 at 20:21

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"I am not getting the "right" answer." - what then were you expecting? –  Ｊ. Ｍ. May 12 '13 at 17:14
I'm getting a long dump of answers when i type f[x]'. I expect to get each exponential multiplied by i*k. I –  olliepower May 12 '13 at 17:16
Really? The output of D[(Exp[I k x] + Exp[-I k x])/2, x] sure looks reasonable to me. –  Ｊ. Ｍ. May 12 '13 at 17:18
you mean f'[x]? –  Spawn1701D May 12 '13 at 17:20
Since your problem is a simple syntax error I'm voting to close. –  Sjoerd C. de Vries May 12 '13 at 17:53

f'[x]

or try ExpToTrig[D[f[x], x]] to see what it really is. –  bill s May 12 '13 at 17:45