f(x)=(ax+b)sinx+(cx+d)cosx试确定常数a,b,c,d使得f`(x)=xcos

f'(x)=asinx+(ax+b)cosx+c*cosx-(cx+d)sinx
=(a+d)sinx-cxsinx+axcosx+(b+c)cosx
恒等于xcosx
=>
a+d=0,c=0,a=1,b+c=0
a=1
b=0
c=0
d=-1