Limits!

Calculus Level 3

lim x 0 ( 1 x 8 + x 3 1 2 x ) \large \lim_{x\to0} \left(\dfrac1{x\sqrt[3]{8+x}} - \dfrac1{2x} \right)

If the limit above can be expressed as a b -\dfrac ab , where a a and b b are coprime positive integers, find a + b \sqrt{a+b} .


The answer is 7.

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3 solutions

Rishabh Jain
Jun 19, 2016

Lets call the limit as F \mathfrak F .

F = lim x 0 2 8 + x 3 2 x 8 + x 3 2 = lim x 0 1 1 + x 8 3 2 x \mathfrak F=\displaystyle\lim_{x\to 0}\dfrac{2-\sqrt[3]{8+x}}{2x\underbrace{\sqrt[3]{8+x}}_2}=\displaystyle\lim_{x\to 0}\dfrac{1-\sqrt[3]{1+\frac x8}}{2x}

Using Binomial approximation 1 + x 8 3 = 1 + x 24 as x 0 \color{#3D99F6}{\small{\sqrt[3]{1+\frac x8}=1+\frac x{24}\text{ as }x\to 0}} and thus F = 1 48 \mathfrak F=\dfrac{-1}{48} 1 + 48 = 7 \Large \therefore\sqrt{1+48}=\boxed{\color{#007fff}{7}}

good solution.. how's mine?

Ayush G Rai - 4 years, 12 months ago

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I'm not able to read it properly... Maybe you can type it in latex

Rishabh Jain - 4 years, 12 months ago

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wait i'll correct it

Ayush G Rai - 4 years, 12 months ago

check it now

Ayush G Rai - 4 years, 12 months ago
Ayush G Rai
Jun 19, 2016

Prince Loomba
Jun 19, 2016

Take x common from denominator and then rationalise the remaining expression to get the answer as 1 48 -\frac{1}{48}

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