Suppose z=x^2*sin(y), x=2s^2−4t^2, y=4st.
AUse the chain rule to find ∂z/∂s and ∂z/∂t as functions of x, y, s and t.
AUse the chain rule to find ∂z/∂s and ∂z/∂t as functions of x, y, s and t.