How do you evaluate the definite integral int sin(4x)sin(4x) from [0,pi/8][0,π8]?

1 Answer
Apr 4, 2018

1/414

Explanation:

This is a fairly common integral.

intsin(ax)dx=-1/acos(ax)sin(ax)dx=1acos(ax) where aa is any constant.

So,

int_0^(pi/8)sin(4x)dx=-1/4cos4x|_0^(pi/8)π80sin(4x)dx=14cos4xπ80

Evaluating, we get

-1/4cos((4pi)/8)-(-1/4cos(0))=-1/4cos(pi/2)+1/4cos(0)=1/414cos(4π8)(14cos(0))=14cos(π2)+14cos(0)=14

As cos0=1cos0=1