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Calculus Level 4

f ( x ) = 1 x x x + 24 t 2 + 1 d t \large f(x)=\dfrac1x\int_x^{x+24}\sqrt{t^2+1}\,dt

Find lim x f ( x ) \displaystyle\lim_{x\to\infty}f(x) .


The answer is 24.

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2 solutions

Kushal Bose
Jul 28, 2016

f ( x ) = x x + 24 t 2 + 1 d t x \large f(x)=\displaystyle \frac{\int_{x}^{x+24} \sqrt{t^2+1} dt}{x}

Put x = x=\infty in both neumerator and denominator..In neumerator evaluate integral by-parts put as limits infinity the value will be tends to infinity.Denominator will also be infinity.

Now differentiate w.r.t. x .For nuemerator use Newton-Liebnitz rule.

f ( x ) = lim x ( x + 24 ) 2 + 1 x 2 + 1 1 f(x)=\displaystyle \lim_{x \to \infty}\frac{\sqrt{(x+24)^2+1} - \sqrt{x^2+1}}{1}

Now substitute x = 1 z x=\frac{1}{z}

lim z 0 ( 1 + 24 z ) 2 + z 2 z 2 + 1 z \displaystyle \lim_{z \to 0} \frac{\sqrt{(1+24z)^2 + z^2}- \sqrt{z^2+1}}{z}

putting z = 0 z=0 in both sides it is in 0 / 0 0/0 form .So using L'hospital Rule differentiating w.r.t. x x

lim z 0 2 ( 1 + 24 z ) 24 + 2 z 2 ( 1 + 24 z ) 2 + z 2 2 z 2 z 2 + 1 \displaystyle \lim_{z \to 0} \frac{2(1+24 z)24 +2z}{2\sqrt{(1+24z)^2 + z^2}} - \frac{2 z}{2\sqrt{z^2+1}}

putting z = 0 z=0 we get the value 24 \boxed{24}

Can you elaborate please? @Kushal Bose

Anik Mandal - 4 years, 8 months ago

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I have updated my solution accordingly .Plz check back and let me know if it is sufficient or not.

Kushal Bose - 4 years, 8 months ago

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Yes it's sufficients.Thanks a lot!

Anik Mandal - 4 years, 8 months ago
Zach Abueg
Oct 22, 2017

L = lim x 1 x x x + 24 t 2 + 1 d t L’hopital’s Rule = lim x d d x x x + 24 t 2 + 1 d t = lim x d d x ( c x + 24 t 2 + 1 d t c x t 2 + 1 d t ) = lim x ( x + 24 ) 2 + 1 x 2 + 1 Multiplying through by ( x + 24 ) 2 + 1 + x 2 + 1 = lim x 48 x + 576 ( x + 24 ) 2 + 1 + x 2 + 1 = lim x 48 x ( 1 + 12 x ) x 1 + 48 x + 577 x 2 + x 1 + 1 x 2 = lim x 48 x 2 x = 24 \begin{aligned} L & = \lim_{x \to \infty} \frac 1x \int_x^{x + 24} \sqrt{t^2 + 1} \ dt & \small \color{#3D99F6} \text{L'hopital's Rule} \\ & = \lim_{x \to \infty} \frac{d}{dx} \int_x^{x+24} \sqrt{t^2 + 1} \ dt \\ & = \lim_{x \to \infty} \frac{d}{dx} \left(\int_c^{x + 24} \sqrt{t^2 + 1} \ dt - \int_c^x \sqrt{t^2 + 1} \ dt \right) \\ & = \lim_{x \to \infty} \sqrt{(x + 24)^2 + 1} - \sqrt{x^2 + 1} & \small \color{#3D99F6} \text{Multiplying through by } \sqrt{(x + 24)^2 + 1} + \sqrt{x^2 + 1} \\ & = \lim_{x \to \infty} \frac{48x + 576}{\sqrt{(x + 24)^2 + 1} + \sqrt{x^2 + 1}} \\ & = \lim_{x \to \infty} \frac{48x\left(1 + \frac{12}{x}\right)}{x\sqrt{1 + \frac{48}{x} + \frac{577}{x^2}} + x\sqrt{1 + \frac{1}{x^2}}} \\ & = \lim_{x \to \infty} \frac{48x}{2x} \\ & = \boxed{24} \end{aligned}

2nd last step is wrong conceptually.

raj abhinav - 1 year ago

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