曲线y=x^2/2与 y^2+x^2=8 交点(-2,2) (2,2)
围成图形的面积=∫(-2~2) [8-x^2]^1/2-x^2/2 dx
=4arcsin[x/(2*2^0.5)]+2^0.5 x (1-x^2/8)^0.5 -x^3/6 上下限(-2~2)
=2Pi + 4/3arcsin是怎么划出来的啊？x=2*2^0.5sint