A while back I saw someone claim that you could prove the product rule in calculus with the single variable chain rule. He provided a proof, but it was utterly incomprehensible. It is easy to prove from the multi variable chain rule, or from logarithmic differentiation, or even from first principles. Is there an actual proof using just the single variable chain rule?
[Math] A proof of the product rule using the single variable chain rule
alternative-proofcalculus
Best Answer
Credit is due to this video by Mathsaurus, but also requires the sum and power rules for derivatives.
Let $u$, $v$ be appropriate functions. Consider $f = (u+v)^2$. By the chain rule, $$f^{\prime} = 2(u+v)(u^{\prime}+v^{\prime})=2(uu^{\prime}+uv^{\prime}+vu^{\prime}+vv^{\prime})\text{.}$$ Now, expand $f$ to obtain $f = u^2+2uv+v^2$, and then we have $$f^{\prime}=2uu^{\prime}+2(uv)^{\prime}+2vv^{\prime}=2[uu^{\prime}+(uv)^{\prime}+vv^{\prime}]\text{.}$$ It follows immediately that $$(uv)^{\prime}=uv^{\prime}+vu^{\prime}\text{.}$$