3. 設 \( w = f\left(ts, \frac{s}{t^2}\right) \), \(\frac{\partial f}{\partial x}(x,y) = x^2y\), \(\frac{\partial f}{\partial y}(x,y) = y^2\)。求偏導數 \(\frac{\partial w}{\partial s}\) 與 \(\frac{\partial w}{\partial t}\) 在點 \((s,t) = (-1,1)\) 的值。