Use the chain rule to find \frac{\partial z}{\partial s} and \frac{\partial z}{\partial t}
a) z = e^{xy} \tan(y) where x = s + 2t and y = s/t
b) z = \sin(\alpha)\sin(\beta) where \alpha = 3s + t and \beta = s - t
c) z = x^2 + y^2 + w^2 where x = st, y = s\cos(t), w = s\sin(t)
