I=∫(0,2)（2-x^2）^（1/2） dx
let
x= √2sina
dx=√2cosada
x=0,a=0
x=2,a=π/4
I=∫(0,π/4)2(cosa)^2 da
=∫(0,π/4)(1+cos2a) da
= [a + (sin2a)/2](0,π/4)
= π/4 +1