Functions of 2 or more Variables

Question 1

What is the equation for a change of variable? Say z = f(x,y) and x = x(s,t), y = y(s,t). (Want ∂z/∂t)

Answer 1

∂z/∂t = ∂z/∂x.∂x/∂t + ∂z/∂y.∂y/∂t