Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

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:

$$9(\frac{1}{2}\sin(2x)+\cos(x))|_{0}^{{\pi}{6}}+9(-\cos(x)-\frac{1}{2}\sin(2x))|_\frac{\pi}{6}^\frac{pi}{2}$$

"Simplifying" I got this:

$$\frac92\sin\left(2\frac\pi6\right)+9\cos\left(\frac\pi6\right)-[\frac92\sin(2\cdot0)+9\cos(0)]-9\cos\left(\frac\pi2\right)-\frac92\sin(\pi)-[-9\cos\left(\frac\pi6\right)-\frac92\sin\left(\frac\pi3\right)]$$

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

$$\frac{9\sqrt3}4+\frac{9\sqrt3}2-[0+9]-0+\frac92-[-\frac{9\sqrt3}2-\frac{9\sqrt3}4]$$

I got a final answer of:

$$\frac{9\sqrt3}2+9\sqrt3+\frac92-9$$

Please help me :(

share|improve this question
    
$ \ \frac{9}{2} \cdot \sin \pi = \ $ ?! –  RecklessReckoner Aug 30 '13 at 2:54
    
@RecklessReckoner $\frac{-9}{2}$? –  inquisitor Aug 30 '13 at 2:56
    
Sine of pi is what? –  RecklessReckoner Aug 30 '13 at 2:56
    
@RecklessReckoner omg.. it's 0 –  inquisitor Aug 30 '13 at 2:57
1  
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

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.