先把前两项或者后两项放一起 ，y'=(x^2)'lnxcosx+x^2(lnxcosx)'=2xlnxcosx+x^2(lnx'cosx+lnxcosx')=2xlnxcosx+x^2(1/xcosx-lnxsinx)=2*x*ln(x)*cos(x)+x*cos(x)-x^2*ln(x)*sin(x)
