等式两边对x 求导得 xf(x)=3x^2*lnx+x^2
∴f(x)=3xlnx+x
两边积分得
∫f(x)dx=3∫xlnxdx+∫xdx
=(3/2)∫lnxd(x^2)+(1/2)x^2
=(3/2)x^2*lnx-(3/2)∫xdx+(1/2)x^2
=(3/2)x^2-(1/4)x^2+C