一个半圆在-2到2上的面积
∫(0->1) 2x dx=[x^2]|(0->1)=1letx=2sinudx=2cosu dux=0, u=0x=2 , u=π/2∫(-2->2) √(4-x^2) dx=2∫(0->2) √(4-x^2) dx=8∫(0->π/2) (cosu)^2 du=4∫(0->π/2) (1+cos2u) du=4[u+(1/2)sin2u]|(0->π/2)=2π
