The title could be How to Find the Derivative of f(x) = sinxcosx?

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SUMMARY

The derivative of the function f(x) = sin(x)cos(x) can be calculated using the Product Rule, resulting in f'(x) = cos^2(x) - sin^2(x) or equivalently f'(x) = (1/2)sin(2x). The initial attempt at differentiation led to an incorrect conclusion that the derivative equals zero, which is not valid as sin(x)cos(x) is not a constant function. Recognizing trigonometric identities is crucial for simplifying the derivative correctly.

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  • Basic differentiation techniques
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Homework Statement



f(x) = sinxcosx

Homework Equations



Product Rule: f(x)g(x) = f(x)Dg(x) + g(x)Df(x)

The Attempt at a Solution



I got to sinx(-sinx) + cosxcosx

The answer is supposed to be 0

Would the next step be turning it into -(sinx)2 + (cosx)2?
 
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Your work is right.

It can't be zero, because otherwise, you could integrate it and you'd obtain a constant, and clearly sinxcosx is not a constant. It's actually (sin 2x)/2
 
Your answer is right (both forms).

Another way to solve this is to recognize that (as l'Hôpital said):

\sin x \cos x = \frac{1}{2} \sin 2x

which has derivative

\cos 2x

The answer looks different, but remembering one's trig identities pays dividends:

\cos 2x = \cos^2 x - \sin^2 x
 

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