Calculus 4

Calculus Level 3

lim x 2 x + 3 x 3 + 5 x 5 3 x 2 + 2 x 3 3 \large \lim_{x\to \infty} \dfrac{2\sqrt x + 3\sqrt[3]{x} +5 \sqrt[5]{x}}{\sqrt{3x-2} + \sqrt[3]{2x-3}}

The limit above has a closed form. Find the value of this closed form to 3 decimal places.


The answer is 1.154.

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2 solutions

Sabhrant Sachan
Jun 16, 2016

lim x 2 x + 2 x 3 + 5 x 5 3 x 2 + 2 x 3 3 lim x 2 + 2 x 1 6 + 5 x 3 10 3 2 x + 2 x 1 2 3 x 3 2 3 2 3 1.154 \displaystyle\lim_{x \to \infty} \dfrac{2\sqrt{x}+2\sqrt[3]{x}+5\sqrt[5]{x}}{\sqrt{3x-2}+\sqrt[3]{2x-3}} \\ \quad \\ \displaystyle\lim_{x \to \infty} \dfrac{2+\dfrac{2}{x^{\frac16}}+\dfrac{5}{x^{\frac3{10}} }}{\sqrt{3-\dfrac{2}{x}}+\sqrt[3]{\dfrac{2}{x^{\frac12}}-\dfrac{3}{x^{\frac32}} }} \implies \dfrac{2}{\sqrt3} \implies \boxed{1.154}

I did it exactly the same way as yours. +1

Akshay Yadav - 4 years, 12 months ago

The approximation of 3 1.7 \sqrt{3} \approx 1.7 is terrible. The decimal answers have a 2% margin of error, so your approximations should be within that range too. Note that we could have calculated it as 2 3 3 \frac{2 \sqrt{3} }{3} , which under your approximation would yield 3.4 3 = 1.13 \frac{ 3.4}{3} = 1.13 .

I have removed the approximation from the question. Can you update​ your answer?

Calvin Lin Staff - 4 years, 11 months ago

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No problem sir ☺.

Sabhrant Sachan - 4 years, 11 months ago

This question is not for level 4 its max for level 2

Sachin Sharma - 4 years, 11 months ago
Ayush G Rai
Jun 17, 2016

In the numerator we must neglect 3 x 3 + x 5 3\sqrt[3]{x}+\sqrt[5]{x} as 2 x 2\sqrt x is much much greater compared to it as x x approaches . \infty. In the denominator,we must neglect 2 x 3 3 \sqrt[3]{2x-3} as 3 x 2 \sqrt {3x-2} is much much greater compared to it as x x approaches . \infty. Now the problem reduces to, lim x 2 x 3 x 2 . \lim_{x\to \infty} \frac{2\sqrt x}{\sqrt {3x-2}}. We again can neglect 2 -2 in the denominator as much much smaller compared to 3 x . 3x.
So, lim x 2 x 3 x \lim_{x\to \infty}\frac{2\sqrt x}{\sqrt{3x}} = 2 3 = 1.154 . =\frac{2}{\sqrt3}=\boxed {1.154}.

The approximation of 3 1.7 \sqrt{3} \approx 1.7 is terrible. The decimal answers have a 2% margin of error, so your approximations should be within that range too. Note that we could have calculated it as 2 3 3 \frac{2 \sqrt{3} }{3} , which under your approximation would yield 3.4 3 = 1.13 \frac{ 3.4}{3} = 1.13 .

I have removed the approximation from the question. Can you update your answer?

Calvin Lin Staff - 4 years, 11 months ago

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ok calvin sir.

Ayush G Rai - 4 years, 11 months ago

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