∫x²e^x²dx
=(1/2)∫xe^x²d(x²)=(1/2)∫xd(e^x²)
=(1/2)xe^x²-(1/2)∫e^x²dx
=(1/2)xe^x²-(√π/4)*erfi(x)+c
其中erfi(x)是虚误差函数