原式=∫1/2 xlnxdx
=1/4 ∫lnxd(x^2)
=1/4[x^2lnx-∫x^2(1/x)dx]
=1/4[x^2lnx-∫xdx]
=(1/4)x^2lnx -x^2/8+c,c为常数