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**[SOLVED] Integral of {sqrt(4-x^2)} from 0 to 2?**

Can someone please explain step by step for this?

I know the answer is pi, I just don't understand the steps towards solving it

First, I'm supposed to set x = 2sin(u)

so then u = arcsin(x/2)

then somehow, {sqrt(4-x^2)} = 2cos(u)

How'd that happen? @__@

I'm really really confused!

Thanks for tolerating XD