Two Undefined Limits?

Calculus Level 1

lim x 2 ( 2 x 2 + 1 x + 2 8 x 2 4 ) = ? \large\displaystyle\lim_{x\to2}\left(\dfrac2{x-2}+\dfrac1{x+2}-\dfrac8{x^2-4}\right)=\, ?


The answer is 0.75.

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4 solutions

Rishabh Jain
Mar 24, 2016

Combine the three fractions together into one fraction:

lim x 2 ( 3 x 6 x 2 4 ) \large\displaystyle\lim_{x\to2}\left(\dfrac{3x-6}{x^2-4}\right) = lim x 2 ( 3 ( x 2 ) ( x + 2 ) ( x 2 ) ) \large =\displaystyle\lim_{x\to2}\left(\dfrac{3\cancel{(x-2)}}{(x+2)\cancel{(x-2)}}\right) = 3 4 = 0.75 \huge =\dfrac{3}{4}=\boxed{0.75}

Ninja skills.

Mehul Arora - 5 years, 2 months ago

In general for computing limit lim x 2 ( 2 x 2 + 1 x + 2 8 x 2 4 ) = lim x 2 ( 2 ( 2 ) 2 + 1 ( 2 ) + 2 8 ( 2 ) 2 4 ) = 0.75 \begin{aligned} \lim_{x \to 2}\left(\frac{2}{x-2}+\frac{1}{x+2}-\frac{8}{x^2-4}\right) &= \lim_{x \to 2}\left(\frac{2}{(2)-2}+\frac{1}{(2)+2}-\frac{8}{(2)^2-4}\right) \\&= 0.75 \space\space\ \square \end{aligned}

FIN!!! \large \text{FIN!!!}

It is easy to see that 8 ( x + 2 ) ( x 2 ) \dfrac{8}{(x+2)(x-2)} can be written as 2 x 2 2 x + 2 \dfrac{2}{x-2}\ - \dfrac{2}{x+2} . Therefore, the terms with x-2 at the denominator cancel and we are left with lim x 2 3 x + 2 \large \lim_{x\to2} \dfrac{3}{x+2} , which is obviously 3 4 \dfrac{3}{4}

Since x x approaches 2 2 , substitute a value for x x that is very near to 2 2 , let x = 1.9999 x=1.9999 . Substitute then use a calculator, so we have

2 1.9999 2 + 1 1.9999 + 2 8 1.999 9 2 4 0.75 \dfrac{2}{1.9999-2}+\dfrac{1}{1.9999+2}-\dfrac{8}{1.9999^2-4} \approx \boxed{0.75}

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