Question

Can someone explain to me where the 2 (just before the integral) in Step 4, originated from, and why the bounds are now [0, pi/3] and not [-pi/3, pi/3] anymore?

(See photo below)

Answer 1

So, to find the area, we have to take the top function minus the bottom function

Answer 2

But notice how it's symmetrical to the y-axis

Answer 3

Say we want to integral x^2.

we can do