Quotient Rule

In calculus, the quotient rule is a method of finding the derivative of a function which is the quotient of two other functions for which derivatives exist. If the function one wishes to differentiate, f(x), can be written as
f(x) = \frac{g(x)}{h(x)}
and h(x) ≠ 0; then, the rule states that the derivative of g(x) / h(x) is equal to the denominator times the derivative of the numerator, minus the numerator times the derivative of the denominator, all divided by the square of the denominator:
f'(x)=\frac{g'(x)h(x) - g(x)h'(x)}{\Delta x}
= \lim_{\Delta x \to 0} \frac{1}{\Delta x} \leftx)h(x)-g(x)h(x+\Delta x)}{h(x)h(x+\Delta x)} \right
= \lim_{\Delta x \to 0} \frac{1}{\Delta x} \leftx)h(x)-g(x)h(x)-x)-g(x)h(x)}{h(x)h(x+\Delta x)} \right]
= \lim_{\Delta x \to 0} \frac{1}{\Delta x} \leftx)-g(x)-g(x)x)-h(x)}{h(x)h(x+\Delta x)} \right]
= \lim_{\Delta x \to 0} \frac{\frac{g(x+\Delta x)-g(x)}{\Delta x}h(x)-g(x)\frac{h(x+\Delta x)-h(x)}{\Delta x}}{h(x)h(x+\Delta x)}
= \frac{\lim_{\Delta x \to 0} \left(\frac{g(x+\Delta x)-g(x)}{\Delta x}\right)h(x) - g(x) \lim_{\Delta x \to 0} \left(\frac{h(x+\Delta x)-h(x)}{\Delta x}\right)}{h(x)h(\lim_{\Delta x \to 0} (x+\Delta x))}
= \frac{g'(x)h(x) - g(x)h'(x)}{h(x)^2}

From the Product Rule

\mbox{let }f(x)=\frac{g(x)}{h(x)}
g(x)=f(x)h(x)\mbox{ }
g'(x)=f'(x)h(x) + f(x)h'(x)\mbox{ }
The rest is simple algebra to make f'(x) the only term on the left hand side of the equation and to remove f(x) from the right side of the equation.
f'(x)=\frac{g'(x) - f(x)h'(x)}{h(x)} = \frac{g'(x) - \frac{g(x)}{h(x)}\cdot h'(x)}{h(x)}
f'(x)=\frac{g'(x)h(x) - g(x)h'(x)}{\left(h(x)\right)^2}

Mnemonic

It is often memorized as a rhyme type song. "Lo-dee-hi minus hi-dee-lo all over lo-lo"; Lo being the denominator, Hi being the numerator and D being the derivative.

 

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