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Classifying the discontinuity of a rational-radical function

Consider the function e(x)=x+5−3x−4.e(x) = \frac{\sqrt{x+5} - 3}{x - 4}. Note that e(4)e(4) is undefined since the denominator is zero there. Determine lim⁡x→4e(x)\lim_{x \to 4} e(x), and use it to classify the discontinuity of e(x)e(x) at x=4x = 4 as removable, jump, or infinite.

Hint

Rationalize the numerator by multiplying by x+5+3x+5+3\frac{\sqrt{x+5}+3}{\sqrt{x+5}+3}, then simplify and cancel the common factor of (x−4)(x-4) before taking the limit.

Step-by-step solution

Multiply numerator and denominator by the conjugate x+5+3\sqrt{x+5}+3: e(x)=(x+5−3)(x+5+3)(x−4)(x+5+3)=(x+5)−9(x−4)(x+5+3)=x−4(x−4)(x+5+3).e(x) = \frac{(\sqrt{x+5}-3)(\sqrt{x+5}+3)}{(x-4)(\sqrt{x+5}+3)} = \frac{(x+5)-9}{(x-4)(\sqrt{x+5}+3)} = \frac{x-4}{(x-4)(\sqrt{x+5}+3)}. For x≠4x \neq 4, the factor (x−4)(x-4) cancels, leaving e(x)=1x+5+3.e(x) = \frac{1}{\sqrt{x+5}+3}.

Now take the limit of the simplified expression as x→4x \to 4: lim⁡x→41x+5+3=19+3=16.\lim_{x\to 4} \frac{1}{\sqrt{x+5}+3} = \frac{1}{\sqrt{9}+3} = \frac{1}{6}. Since this limit exists and is finite but e(4)e(4) is undefined, the graph of e(x)e(x) has a hole at x=4x=4 that could be patched by defining e(4)=16e(4)=\frac{1}{6}. This is a removable discontinuity.

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