y=(1-x^2)^0.5 y=x x=0 围成 :1/8的单位圆
极坐标方程:R≤1,(0≤θ≤π/4)
I=∫∫(1-y^2)^0.5 dxdy =∫dθ∫RCOSθdR 0≤R≤1,0≤θ≤π/4
=根号2/4