∫∫e^(x^2+y^2)dxdy=∫[0->2π]∫[0->2]e^(r^2)rdrdθ
=π∫[0->2]e^(r^2)d(r^2)
=π[e^(r^2)] | [0->2]
=π(e^4-1)这个就是用的极坐标啊