交换积分顺序∫[0,1]dx∫[x,1]e^(-y^2)dy=∫[0,1]dy∫[y,0]e^(-y^2)dx=∫[0,1]ye^(-y^2)dy=(1/2)∫[0,1]e^(-y^2)d(y^2)=(1/2)[-e^(-y^2)] | [0->1]=(1/2)[1-e^(-1)]
