如图
由x=t*e^t得：dx/dt = e^t + t*e^t由e^t +e^y = 2e得：dy/dt = -e^t/e^y = -e^t/(2e-e^t)当t=1时，dx/dt=2e，dy/dt = -1所以，k = dy/dx = (dy/dt)/(dx/dt) = -1/(2e)
