即u=e^t *(sint -cost)那么求导得到du/dt=(e^t)' *(sint -cost) +e^t *(sint -cost)'显然(e^t)'=e^t,而(sint -cost)'=cost+sint所以得到du/dt=e^t *(sint -cost+cost+sint)=2e^t * sint
u=e^t(sint-cost) =e^tsint-e^tcost 求导得=2e^tsint