Limits

Calculus Level 2

Evaluate the limit.

L = lim x 0 ( x + 4 ) 3 2 + e x 9 x L=\large \lim_{x\rightarrow 0}\frac{(x+4)^{\frac{3}{2}}+e^{x}-9}{x}

Bonus : Solve this limit without using L'hopital's rule .


The answer is 4.

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3 solutions

First , transformed into an algebraic limit

Now using standard following theorems ,

lim x 0 e x 1 x = 1 , lim x a x n a n x a = n a n 1 \lim_{x \to 0}\frac{e^{x} - 1}{x} = 1,\,\lim_{x \to a}\frac{x^{n} - a^{n}}{x - a} = na^{n - 1}

We get ,

L = lim x 0 ( x + 4 ) 3 / 2 + e x 9 x = lim x 0 ( x + 4 ) 3 / 2 8 x + e x 1 x = lim x 0 ( x + 4 ) 3 / 2 8 ( x + 4 ) 4 + 1 = lim t 4 t 3 / 2 4 3 / 2 t 4 + 1 (by putting t = x + 4 ) = 3 2 4 1 / 2 + 1 = 4 \begin{aligned}L &= \lim_{x \to 0}\frac{(x + 4)^{3/2} + e^{x} - 9}{x}\\ &= \lim_{x \to 0}\frac{(x + 4)^{3/2} - 8}{x} + \frac{e^{x} - 1}{x}\\ &= \lim_{x \to 0}\frac{(x + 4)^{3/2} - 8}{(x + 4) - 4} + 1\\ &= \lim_{t \to 4}\frac{t^{3/2} - 4^{3/2}}{t - 4} + 1\text{ (by putting }t = x + 4)\\ &= \frac{3}{2}\cdot 4^{1/2} + 1 = 4\end{aligned}

This approach is without L Hosp. rule. View @Svatejas Shivakumar 's solution for the bonus one.

Chew-Seong Cheong
Feb 14, 2016

L = lim x 0 ( x + 4 ) 3 2 + e x 9 x = lim x 0 8 ( 1 + x 4 ) 3 2 + e x 9 x Using Maclaurin series = lim x 0 8 ( 1 + 3 x 8 + 3 x 2 128 x 3 1024 . . . ) + ( 1 + x + x 2 2 + x 3 6 + . . . ) 9 x = lim x 0 ( 8 + 3 x + 3 x 2 16 x 3 128 . . . ) + ( 1 + x + x 2 2 + x 3 6 + . . . ) 9 x = lim x 0 4 x + 11 16 x 2 + 61 384 x 3 + . . . x = lim x 0 4 + 11 16 x + 61 384 x 2 + . . . = 4 \begin{aligned} L & = \lim_{x \to 0} \frac{\color{#3D99F6}{(x+4)^{\frac{3}{2}}}+\color{#D61F06}{e^x}-9}{x} \\ & = \lim_{x \to 0} \frac{\color{#3D99F6}{8(1+\frac{x}{4})^{\frac{3}{2}}}+\color{#D61F06}{e^x}-9}{x} \quad \quad \small \color{#3D99F6}{\text{Using Maclaurin series}} \\ & = \lim_{x \to 0} \frac{\color{#3D99F6}{8\left(1+\frac{3x}{8}+\frac{3x^2}{128} - \frac{x^3}{1024}-... \right)}+\color{#D61F06}{\left(1+x+\frac{x^2}{2}+\frac{x^3}{6}+...\right)}-9}{x} \\ & = \lim_{x \to 0} \frac{\color{#3D99F6}{\left(8+3x+\frac{3x^2}{16} - \frac{x^3}{128}-... \right)}+\color{#D61F06}{\left(1+x+\frac{x^2}{2}+\frac{x^3}{6}+...\right)}-9}{x} \\ & = \lim_{x \to 0} \frac{4x + \frac{11}{16}x^2 + \frac{61}{384}x^3+...}{x} \\ & = \lim_{x \to 0}\space 4 + \frac{11}{16}x + \frac{61}{384}x^2+... \\ & = \boxed{4} \end{aligned}

Nice approach ...

A Former Brilliant Member - 5 years, 4 months ago

Substituting x = 0 x=0 , L L takes the form of 0 0 \frac{0}{0} , the derivative of L L is 3 2 × ( x + 4 ) 1 / 2 \frac{3}{2} \times (x+4)^{1/2} . By L'Hopital's Rule substituting the derivative of L L with x = 0 x=0 we get the required limit as 4 \boxed{4}

You were not supposed to use L Hopitals rule.

asad bhai - 5 years, 4 months ago

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That's a bonus question and this is the solution for the actual question.

Nihar Mahajan - 5 years, 3 months ago

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