∫（2,1）xlnxdx=∫（2,1）(1/2)lnxd(x^2)=(1/2)x^2lnx-(1/2)∫（2,1）xdx=(1/2)[x^2lnx-(1/2)x^2]（2,1）=(1/2)(4ln2-3/2)=2ln2-3/4
