Corollary 4. The derivative (g o f)'(x) of the composition of differentiable
real-valued functions equals the product g
f
(f(x)) • f'(x) of the derivatives of
these functions computed at the corresponding points.
There is a strong temptation to give a short proof of this last statement
in Leibniz' notation for the derivative, in which if z = z(y) and y = y(x), we
have
dz dz d