Indeterminate form?

Calculus Level 3

lim x ( 8 x + 3 x 3 4 x 2 x ) = ? \large \displaystyle \lim_{x\to\infty}\left(\sqrt[3]{8^x+3^x}-\sqrt{4^x-2^x} \right)= \, ?


Inspiration .


The answer is 0.5.

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3 solutions

Chew-Seong Cheong
May 17, 2016

L = lim x ( 8 x + 3 x 3 4 x 2 2 ) = lim x ( 2 x [ 1 + ( 3 8 ) x ] 1 3 2 x [ 1 ( 2 4 ) x ] 1 2 ) By binomial expansion = lim x ( 2 x [ 1 + 1 3 ( 3 8 ) x 1 9 ( 3 8 ) 2 x + . . . ] 2 x [ 1 1 2 ( 1 2 ) x 1 8 ( 1 2 ) 2 x . . . ] ) = lim x ( [ 2 x + 3 x 1 2 2 x 3 2 x 2 2 5 x + . . . ] [ 2 x 1 2 1 2 x + 3 . . . ] ) 2 x 2 x = 0 and all red terms 0 as x = 1 2 = 0.5 \begin{aligned} \mathfrak L & = \lim_{x \to \infty} \left( \sqrt[3]{8^x+3^x} - \sqrt{4^x-2^2} \right) \\ & = \lim_{x \to \infty} \left( 2^x \left[1+ \left(\frac{3}{8}\right)^x \right]^\frac{1}{3} - 2^x \left[1 - \left(\frac{2}{4}\right)^x \right]^\frac{1}{2} \right) \quad \quad \small \color{#3D99F6}{\text{By binomial expansion}} \\ & = \lim_{x \to \infty} \left(2^x \left[1+ \frac{1}{3}\left(\frac{3}{8}\right)^x - \frac{1}{9}\left(\frac{3}{8}\right)^{2x} + ... \right] - 2^x \left[1 - \frac{1}{2}\left(\frac{1}{2}\right)^x - \frac{1}{8}\left(\frac{1}{2}\right)^{2x} - ... \right] \right) \\ & = \lim_{x \to \infty} \left(\left[\color{#3D99F6}{2^x} + \color{#D61F06}{\frac{3^{x-1}}{2^{2x}} - \frac{3^{2x-2}}{2^{5x}} + ... } \right] - \left[\color{#3D99F6}{2^x} - \frac{1}{2} - \color{#D61F06}{\frac{1}{2^{x+3}} - ...} \right] \right) \quad \quad \small \color{#3D99F6}{2^x - 2^x = 0} \text{ and }\color{#D61F06}{\text{all red terms}\to 0 \text{ as }x \to \infty} \\ & = \frac{1}{2} = \boxed{0.5} \end{aligned}

Yes! Very clear explanation.

展豪 張 - 5 years ago

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Thanks for the comments. I will do my best.

Chew-Seong Cheong - 5 years ago

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When I write solutions I never think of using different colors... I should learn from you.

展豪 張 - 5 years ago

What happened from line 2 to line 3? Is that a Taylor polynomial

Jerry McKenzie - 4 years ago

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Yes, Taylor expansion. You may want to use this Taylor series calculator .

Chew-Seong Cheong - 4 years ago
Sabhrant Sachan
May 16, 2016

This problem is similar to Iikkyu San's problem. Waiting for Rishabh to post the answer.

Yes. I have just finished that problem. Maybe I'd better quote that as an inspiration.

展豪 張 - 5 years, 1 month ago

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