这个用极坐标令x=pcosa,y=psinaa∈[0，π/2]p∈[0,1]代入得原积分=∫[0，π/2]∫[0,1]√(1-p^2)*pdpda=∫[0，π/2]da∫[0,1]√(1-p^2)*pdp=π/2*(-1/2)∫[0,1]√(1-p^2)d(1-p^2)=π/2*(-1/3)(1-p^2)^(3/2)[0,1]=π/6
