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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 am not getting the "right" answer." - what then were you expecting? – J. M.♦ May 12 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 at 17:16
1  
Really? The output of D[(Exp[I k x] + Exp[-I k x])/2, x] sure looks reasonable to me. – J. M.♦ May 12 at 17:18
2  
you mean f'[x]? – Spawn1701D May 12 at 17:20
Since your problem is a simple syntax error I'm voting to close. – Sjoerd C. de Vries May 12 at 17:53

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

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1 Answer

up vote 3 down vote accepted

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:

f'[x]
D[f[x], x]
Dt[f[x], x] // TraditionalForm
Dt[f[x], x] // FullSimplify // TraditionalForm

Check the documentation for D and Dt and pay close attention to syntax (ie. where all the punctuation and arguments go)

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

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