(x^2*cosx)'=2xcosx-x^2*sinx应用的公式:(ab)=a'b+ab'
y'=2x*cosx-x^2*sinx
y'=(x^2)'*cos x+x^2*(cos x)'=2xcos x-x^2*sin x
2*x*cos(x)-(x^2)*sin(x)
