# Math Help - Trigonometry Integration

1. ## Trigonometry Integration

How do I do this:

$\displaystyle\int (\sin 2x.\sin x) dx$

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The Answer is:
$\frac {2}{3} \sin^3x + c$

Thank you advance.

2. $\sin (2x)\sin (x)dx = 2\sin ^2 (x)\left[ {\cos (x)dx} \right]$

3. Originally Posted by Plato
$\sin (2x)\sin (x)dx = 2\sin ^2 (x)\left[ {\cos (x)dx} \right]$

How is cosx cancelled out? When I use the cosx double angle rule to integrate sinx, I don't obtain the correct answer. Can you show the stages please?

4. No I will not do the problem for you.
But what you have is $2 u^2 du$.

5. Originally Posted by Air
How is cosx cancelled out? When I use the cosx double angle rule to integrate sinx, I don't obtain the correct answer. Can you show the stages please?
$sin(2x)=2sin(x)cos(x)$