积分0到1 √（1-x²） dx
let
x = sina
dx = cosa da
x =0,a= 0
x=1,a =π/2
积分0到1 √（1-x²） dx
= 积分0到 π/2 (cosa)^2 da
= 积分0到 π/2 (cos2a+1)/2 da
=[ (sin2a)/4 + a/2] (0,π/2)
= π/4