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For each function f, find the limit as x approaches c of the average rate of change of f from c to x. That is, find limxcf(x)f(c)xc\lim _{x \rightarrow c} \frac{f(x)-f(c)}{x-c}. f(x)=2x,c=5f(x)=\sqrt{2 x}, c=5

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We would like to find the value of the average rate of change of limxcf(x)f(c)xc\color{#4257b2}\lim\limits_{x \to c}\dfrac{f(x)-f(c)}{x-c} if we know that f(x)=2x\color{#4257b2}f(x)=\sqrt{2x} and c=5\color{#4257b2}c=5. First, we will find the average rate of change of f\color{#4257b2}f from c=5\color{#4257b2}c=5 to x\color{#4257b2}x and then find the limit of this average rate of change.

f(x)f(c)xc=2xf(5)x5=2x2(5)x5=2x10x5\dfrac{f(x)-f(c)}{x-c}=\dfrac{\sqrt{2x}-f(5)}{x-5}=\dfrac{\sqrt{2x}-\sqrt{2(5)}}{x-5}=\dfrac{\sqrt{2x}-\sqrt{10}}{x-5}

Now we have the average rate of change, so the next step is to find the limit of this average as follows:

limxcf(x)f(c)xc=limx52x10x5\lim\limits_{x \to c} \dfrac{f(x)-f(c)}{x-c}=\lim\limits_{x \to 5} \dfrac{\sqrt{2x}-\sqrt{10}}{x-5}

Now we note that the value of the limit of the denominator at x=5\color{#4257b2}x=5 will equal zero, so we need to use another strategy to find the value of the limit. We can multiply the numerator and denominator by 2x+10\color{#4257b2}\sqrt{2x}+\sqrt{10}.

limxcf(x)f(c)xc=limx52x10x5=limx52x10x52x+102x+10=limx5(2x)2(10)2(x5)(2x+10)=limx52x10(x5)(2x+10)=limx52(x5)(x5)(2x+10)=limx52(x5)(x5)(2x+10)=limx522x+10\begin{align*} \lim\limits_{x \to c} \dfrac{f(x)-f(c)}{x-c}&=\lim\limits_{x \to 5} \dfrac{\sqrt{2x}-\sqrt{10}}{x-5} \\ \\ &=\lim\limits_{x \to 5} \dfrac{\sqrt{2x}-\sqrt{10}}{x-5}\cdot \dfrac{\sqrt{2x}+\sqrt{10}}{\sqrt{2x}+\sqrt{10}} \\ \\ &=\lim\limits_{x \to 5} \dfrac{\left(\sqrt{2x}\right)^{2}-\left(\sqrt{10}\right)^{2}}{(x-5)\left(\sqrt{2x}+\sqrt{10}\right)} \\ \\ &=\lim\limits_{x \to 5} \dfrac{2x-10}{(x-5)\left(\sqrt{2x}+\sqrt{10}\right)} \\ \\ &=\lim\limits_{x \to 5} \dfrac{2\left(x-5\right)}{(x-5)\left(\sqrt{2x}+\sqrt{10}\right)} \\ \\ &=\lim\limits_{x \to 5} \dfrac{2\cancel{\left(x-5\right)}}{\cancel{(x-5)}\left(\sqrt{2x}+\sqrt{10}\right)}=\lim\limits_{x \to 5} \dfrac{2}{\sqrt{2x}+\sqrt{10}} \end{align*}

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