logarithmic proof of quotient rule

Following is a proof of the quotient rule using the natural logarithm, the chain rule, and implicit differentiation. Note that circular reasoning does not occur, as each of the concepts used can be proven independently of the quotient rule.

Proof.

Let $f$ and $g$ be differentiable functions and $\displaystyle y=\frac{f(x)}{g(x)}$ . Then $\ln y=\ln f(x)-\ln g(x)$ . Thus, $\displaystyle\frac{1}{y}\cdot\frac{dy}{dx}=\frac{f^{\prime}(x)}{f(x)}-\frac{g^% {\prime}(x)}{g(x)}$ . Therefore,

$\begin{array}[]{rl}\displaystyle\frac{dy}{dx}&\displaystyle=y\left(\frac{f^{% \prime}(x)}{f(x)}-\frac{g^{\prime}(x)}{g(x)}\right)\\ &\\ &\displaystyle=\frac{f(x)}{g(x)}\left(\frac{f^{\prime}(x)}{f(x)}-\frac{g^{% \prime}(x)}{g(x)}\right)\\ &\\ &\displaystyle=\frac{f^{\prime}(x)}{g(x)}-\frac{f(x)g^{\prime}(x)}{(g(x))^{2}}% \\ &\\ &\displaystyle=\frac{g(x)f^{\prime}(x)-f(x)g^{\prime}(x)}{(g(x))^{2}}.\end{array}$

∎

Once students are familiar with the natural logarithm, the chain rule, and implicit differentiation, they typically have no problem following this proof of the quotient rule. Actually, with some prompting, they can produce a proof of the quotient rule to this one. This exercise is a great way for students to review many concepts from calculus.

Title	logarithmic proof of quotient rule
Canonical name	LogarithmicProofOfQuotientRule
Date of creation	2013-03-22 16:18:51
Last modified on	2013-03-22 16:18:51
Owner	Wkbj79 (1863)
Last modified by	Wkbj79 (1863)
Numerical id	7
Author	Wkbj79 (1863)
Entry type	Proof
Classification	msc 26A06
Classification	msc 97D40
Related topic	LogarithmicProofOfProductRule