How to find the integral \int_0^{70\pi}|\cos^2x\sin x|dx

Caitlyn Cole

Caitlyn Cole

Answered question

2022-04-20

How to find the integral
070π|cos2xsinx|dx

Answer & Explanation

Jonas Dickerson

Jonas Dickerson

Beginner2022-04-21Added 22 answers

If you take the integral from 0 to π and then multiply by 70, you've got it, since it's periodic with period π. On the interval from 0 to π you can drop the absolute value since the function is nonnegative there. And
0π2(cos2x)(sinxdx)=10u2(du)
And finally, π2π is the same thing because of geometric symmetry. The bottom line is that your friend is right.
A way of looking at the aforementioned geometric symmetry is this:
π2πcos2xsinxdx=π20cos2(πw)sin(πw)(dw)
=0π2cos2wsinwdw
=0π2cos2xsinxdx
Here we used the trigonometric identities
sin(πw)=sinw
and cos(πw)=cosw
whence cos2(πw)=cos2w

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