转化为极坐标∫∫(1-x^2-y^2 )dσ=∫[0->π/4] dθ∫[0->1] (1-r^2)rdr=-π/8∫[0->1] (1-r^2)d(1-r^2)=(-π/8)*(1/2)*(1-r^2)^2 | [0->1]=π/16
