e^（-lnx+c）=（e^c）/x其实就是e的-lnx次方等于1/x
y = ...... = x [∫x^2e^(∫-dx/x)dx + C] = x [∫x^2e^(-lnx)dx + C]= x [∫x^2dx/e^(lnx) + C] = x [∫x^2dx/x + C]= x [∫xdx + C] = x [(1/2)x^2 + C]
