利用分部积分来积分
f（x）=∫5xsinxdx=-∫5xdcosx=-5xcosx+∫5cosxdx=-5xcosx+∫5dsinx=-5xcosx+5sinx +C
∫5xsinxdx = -5∫xdcosx = -5xcosx + 5∫cosxdx = -5xcosx + 5sinx + C
