частные-производные - Найти $%\frac {dy}{dx}$% и $% \frac {d^2y}{dx^2}$% параметрически заданных функций

$$\eqalign{ & x = {e^t},\,\,\,y = \cos t \cr & {\text{dx/dt}} = {e^t},\,\,\,{\text{dy/dt}} = - \sin t \cr & {\text{dy/dx}} = {\text{dy/dt}} \cdot {\text{dt/dx}} = - \sin t \cdot \frac{1}{{{e^t}}} = - \frac{{\sin t}}{{{e^t}}} \cr & {{\text{d}}^2}{\text{y/d}}{{\text{x}}^2} = {\text{d/dx}} \cdot {\text{(dy/dx)}} = {\text{d/dt}} \cdot {\text{(dy/dx}}) \cdot {\text{dt/dx}} \cr & {{\text{d}}^2}{\text{y/d}}{{\text{x}}^2} = \frac{{\sin t - \cos t}}{{{e^t}}} \cdot \frac{1}{{{e^t}}} = \frac{{\sin t - \cos t}}{{{e^{2t}}}} \cr} $$