(Please think of a pun for LIMIT yourself)

Calculus Level 1

lim x 0 ln ( x + 1 ) x = ? \large{\lim \limits_{x\rightarrow 0} \dfrac{\ln{(x+1)}}{x} = ? }

Limit does not exist. e e 1 -1 ln ( 2 ) \ln{(2)} 0 0 1 1

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4 solutions

James Watson
Aug 3, 2020

Usually L'Hopital's Rule is the way i would go, but i decided to take a different route.

The Taylor Seriers expansion of ln ( x + 1 ) \ln(x+1) at x = 0 x=0 is n = 0 ( 1 ) n + 1 x n n = x x 2 2 + x 3 3 x 4 4 + \displaystyle\sum_{n=0}^{\infty}(-1)^{n+1}\frac{x^n}{n} = x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+\dots

We can plug this into our limit and we can see that: lim x 0 ln ( x + 1 ) x lim x 0 x x 2 2 + x 3 3 x 4 4 + x = lim x 0 1 x 2 + x 2 3 x 3 4 + = 1 0 2 + 0 2 3 0 3 4 + = 1 \begin{aligned} \lim\limits_{x\to 0}\frac{\ln(x+1)}{x} &\Longrightarrow \lim\limits_{x\to 0}\frac{x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+\dots}{x} \\ &= \lim\limits_{x\to 0}1-\frac{x}{2}+\frac{x^2}{3}-\frac{x^3}{4}+\dots \\ &= 1-\frac{0}{2}+\frac{0^2}{3}-\frac{0^3}{4}+\dots = \boxed{1} \end{aligned}

We first substitute x = 0 \text{We first substitute } x=0

lim x 0 ln ( x + 1 ) x = 0 0 \large{\lim \limits_{x\rightarrow 0} \dfrac{\ln{(x+1)}}{x} =\dfrac{0}{0}}

We get a 0 0 situation, so we can apply L’H o ˆ pital’s rule. \text{We get a } \dfrac{0}{0} \text{ situation, so we can apply L'Hôpital's rule.}

We differentiate both numerator and denominator to get: \text{We differentiate both numerator and denominator to get:}

lim x 0 ln ( x + 1 ) x = d d x ( ln ( x + 1 ) ) d d x ( x ) = 1 x + 1 \large{\lim \limits_{x\rightarrow 0} \dfrac{\ln{(x+1)}}{x} = \dfrac{\frac{d}{dx} (\ln{(x+1)})}{\frac{d}{dx} (x)}=\dfrac{1}{x+1}}

Now we can substitute x = 0 and get the answer. \text{Now we can substitute } x=0 \text{ and get the answer.}

lim x 0 ln ( x + 1 ) x = 1 0 + 1 = 1 \large{\lim \limits_{x\rightarrow 0} \dfrac{\ln{(x+1)}}{x} = \dfrac{1}{0+1}=\boxed{1}}

Pun: There is a LIMIT to the amount of limit puns that i can come up with.

James Watson - 10 months, 1 week ago

Uff, again l hopital.

A Former Brilliant Member - 10 months, 1 week ago

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I don't like other methods, they are long :(

Vinayak Srivastava - 10 months, 1 week ago

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You should do those long, rigorous methods.

A Former Brilliant Member - 10 months, 1 week ago

@Kriti Kamal , which method you used? I would love to know!

Vinayak Srivastava - 10 months, 1 week ago

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I am posting it.

A Former Brilliant Member - 10 months, 1 week ago

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@A Former Brilliant Member Why did you delete it?

Vinayak Srivastava - 10 months, 1 week ago

How you do know that the limit of the differentials is the same?

If i take the exponential of top and bottom I get (x +1)/e^x

e^x limits to 1 as x goes to 0

x + 1 limits to 1 as x goes to 0

so this approach gets the answer of 1.

Is this also valid ?

Chris Russell - 10 months, 1 week ago

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Read this wiki on L'Hopital's Rule . It explains why it works for the certain situations

James Watson - 10 months, 1 week ago

How is ln ( 0 + 1 ) = 0 \ln (0+1)=0 ???

Lâm Lê - 9 months ago

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ln 1 = 0 \ln{1} =0

Vinayak Srivastava - 9 months ago

So as you can see, putting x=0 does not help, you become sick, so you need to go to the Hospital. Idk what pun to use.

Stop calling "L'Hôpital" Hospital!!!!!!!

Vinayak Srivastava - 10 months, 1 week ago

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That was the best or weird pun which would be punny(funny) I could think of.

Siddharth Chakravarty - 10 months, 1 week ago

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How can a person's name be "Hospital"?

Vinayak Srivastava - 10 months, 1 week ago

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@Vinayak Srivastava How can a person's surname in India be 'Papadwala','Dabbawala'.

Siddharth Chakravarty - 10 months, 1 week ago

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@Siddharth Chakravarty Let it be! :)

Vinayak Srivastava - 10 months, 1 week ago

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@Vinayak Srivastava Btw, what is your pun approach? :D

Siddharth Chakravarty - 10 months, 1 week ago

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@Siddharth Chakravarty I didn't get anything good, so I wrote this title. :)

Vinayak Srivastava - 10 months, 1 week ago

@Siddharth Chakravarty , 😂 you gave me the reason to use l hopital method.

Let f ( x ) = ln x f(x)=\ln x

Then f ( x ) = 1 x f'(x) =\dfrac 1x

So, f ( 1 ) = 1 1 = 1 = f'(1)=\dfrac 11=1=

lim x 0 ln ( 1 + x ) ln 1 x \displaystyle \lim_{x\to 0} \dfrac {\ln (1+x)-\ln 1}{x}

= lim x 0 ln ( 1 + x ) x =\displaystyle \lim_{x\to 0} \dfrac {\ln (1+x)}{x}

Are you Alak Bhattacharya Sir?

Vinayak Srivastava - 10 months, 1 week ago

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Yes, @Foolish Learner has exact same profile as Mr. Alak @Vinayak Srivastava , but I don't know why he changed to "mention[6594224:Foolish Learner]" even though he is a "very smart learner".

A Former Brilliant Member - 10 months, 1 week ago

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