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I'm so frustrated with this problem! I've spent maybe an hour on it just making sure I have everything perfect since the first couple of times I kept getting it incorrect.

Can you spot my error?

This is the original equation: $$\int_{0}^{\frac{\pi}{2}}|9\sin(x)-9\cos(2x)|dx$$

After doing some factoring and trig identity substitution I came up with the following integrals:

$$9\int_{0}^{\frac{\pi}{6}}\cos(2x) - \sin(x)dx + 9\int_{\frac{\pi}{6}}^{\frac{\pi}{2}}\sin(x) - \cos(2x)dx$$

And then I anti-differentiated:


"Simplifying" I got this:


Simplifying even further I got this (this is where I suspect I messed up):


I got a final answer of:


Please help me :(

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$ \ \frac{9}{2} \cdot \sin \pi = \ $ ?! – RecklessReckoner Aug 30 '13 at 2:54
@RecklessReckoner $\frac{-9}{2}$? – free_mind Aug 30 '13 at 2:56
Sine of pi is what? – RecklessReckoner Aug 30 '13 at 2:56
@RecklessReckoner omg.. it's 0 – free_mind Aug 30 '13 at 2:57
As my old section supervisor would say to me (many years ago now -- with a straight face), "No one has ever made that mistake before..." Little things like that happen to everyone doing math at one time or another -- there's no shame in it. I'm glad that helped. [Take consolation in the fact that you actually did all the hard parts of the calculation correctly!] – RecklessReckoner Aug 30 '13 at 3:01

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