If f is differentiable at the point u = g(x), and g is differentiable at x,
• then the composite function y = f(u) for u = g(x) is differentiable at x and
• then the composite function (fog)(x) = f(g(x)) is differentiable at x and
(fog)'(x) = f′(g(x)) ∗ g′(x)