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Calculus Level 1

Calculate lim x 0 + x ln ( x ) \large \lim\limits_{x\to 0^{+}}-x\ln(x)


The answer is 0.

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5 solutions

James Watson
Jul 23, 2020

lim x 0 + x ln ( x ) = lim x 0 + ln ( x ) 1 x \lim\limits_{x\to 0^{+}}-x\ln(x) = \lim\limits_{x\to 0^{+}}\cfrac{\ln(x)}{\frac{1}{-x}}

We can apply L'Hopital's rule here ( ) \left(\frac{-\infty}{-\infty}\right) and differentiate the top and the bottom to get lim x 0 + 1 x 1 x 2 = lim x 0 + x = 0 \lim\limits_{x\to 0^{+}} \cfrac{\frac{1}{x}}{\frac{1}{x^2}} = \lim\limits_{x\to 0^{+}}x = \boxed{0}

mind me asking, but how do you know differentiation at such a small age.

Justin Arun - 10 months, 3 weeks ago

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After doing some courses and watching a few videos I got the hang of it I guess. There is still lots to learn and master though!

James Watson - 10 months, 3 weeks ago

Well in my country almost everyone at the age of 13-14 knows calculus basics.

Please have a look at my posts as well

Tattwa shiwani - 10 months, 2 weeks ago

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@Tattwa shiwani That is completely untrue. I don't believe you one bit, mate.

Justin Arun - 10 months, 2 weeks ago

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@Justin Arun That's a bit too early but it's true. Though not "almost everyone" knows, Many students preparing for a national examination (JEE) start very early and prepare for year 1 and year 2 topics

Mahdi Raza - 10 months, 2 weeks ago

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@Mahdi Raza Oh ok. It was the "almost everyone" part that I didn't like.

Justin Arun - 10 months, 2 weeks ago

I am puzzled as well...

Mahdi Raza - 10 months, 2 weeks ago

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how come you are puzzled?

James Watson - 10 months, 2 weeks ago

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@James Watson Because I still don't know differentiation even after being the same as your age. I would in about 6 months or so :(

Mahdi Raza - 10 months, 2 weeks ago

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@Mahdi Raza been doing it for a few months now. watching bprp and 3b1b help as well as brilliant courses. also making the puzzles for the community help them feel more natural

James Watson - 10 months, 2 weeks ago

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@James Watson Oo, nice! I should begin as well. I have an intermediate knowledge of limits and indeterminate forms so I know this solution (it's nice BTW). Thanks, keep going

Mahdi Raza - 10 months, 2 weeks ago

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@Mahdi Raza cheers. you too!

James Watson - 10 months, 2 weeks ago

In Spain we learn differential calculus at the age of 16-17

Ismael Nieto - 10 months ago

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cool! only a few of us in England learn it from 15 years old i think but it is a thing in A levels (17 - 18)

James Watson - 10 months ago
Eugen Scorbura
Aug 2, 2020

You can slap everything in the log and get log of the limit as x approaches 0 of 1/x^x. It is a known fact that the limit as x approaches 0 of x^x is 1 so thanks to the log you get 0.

Nice solution! I like how you "slap" everything in the log haha. upvote from me :)

James Watson - 10 months, 2 weeks ago
Java Script
Sep 18, 2020

If we use limits - 0*ln(0), 0 times anything is 0

Even I thought the same & came here to read explanations that how am I correct?!

Akash Rathod - 8 months, 1 week ago

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Yes. We can plug it in since l'hospital doesn't work.

Java Script - 8 months, 1 week ago

What??...No...ln(0) is not defined

Shaumik Khanna - 2 months ago
Sean Elma
Sep 23, 2020

You can use Taylor expansion, if you expend the natural logarithm up to the first order around the point x = lim x-> 0+. You get -5.65 + x you multiply by -x and the result follows

Callie Ferguson
Sep 15, 2020

L’hopital’s rule is:

lim x a f ( x ) g ( x ) = M N \lim_{ x\to\ a} f(x) \cdot g(x) = M \cdot N , where:

M = lim x a f ( x ) M = \lim_{x\to\ a} f(x)

N = lim x a g ( x ) N = \lim_{x\to\ a} g(x)

So in the equation given, we can say that f ( x ) = x f(x)=-x and g ( x ) = l n ( x ) g(x)=ln(x) .

Now, our M M and N N are:

M = lim x 0 + x M = \lim_{x\to\ 0^+} -x

N = lim x 0 + l n ( x ) N = \lim_{x\to\ 0^+} ln(x)

With the limit being as x approaches zero from the right (as positive numbers approach zero).

Plugging in numbers will tell us that M = 0 M=0 , which means that the product M N M \cdot N will be zero regardless of the value of N N .

So, the answer is zero.

What if you were evaluating lim x 0 sin ( x ) tan ( x ) \displaystyle \lim\limits_{x \to 0} \frac{\sin(x)}{\tan(x)} ? Here, your logic implys that M = lim x 0 sin ( x ) M = \lim\limits_{x \to 0} \sin(x) and N = 1 tan ( x ) N =\frac{1}{\tan(x)} and this would result in 0 since lim x 0 sin ( x ) = 0 \lim\limits_{x \to 0}\sin(x) = 0 . However, this limit comes out as 1 in the end so is this method always good?

James Watson - 9 months ago

Well, there are multiple laws for evaluating limits. For the case you mentioned, the first thing I would do would be to simplify the equation to just cos(x), since tan(x)=sin(x)/cos(x). That would then give that the limit is 1, which is correct. There are cases where the method I used wouldn't be the ideal way to solve the problem since the method is tailored to the product of two functions, but simplifying the equation in the first place is definitely important in deciding which method to use. L'hopital was a smart guy - he had a lot of ways of solving limits :)

*Latex wasn't showing up when I posted it so I couldn't use it, sorry!!

Callie Ferguson - 8 months, 4 weeks ago

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@Callie Ferguson I think your solution is a special case because when N tends to ∞ or -∞ (or both) the answer can easily vary some examples being x/tan(1/x) ( does not have a limit at zero ), x/sin(x) ( limit at zero is one ) and x/sin(x^2) ( limit at zero approaches ∞ ), so the product M*N does matter on N even if M=0. None of these functions can be simplified unless you r willing to work with their Taylor expansions ( I think they can’t be simplified without Taylor expansion, please correct me if I am wrong ) ( I don’t know how to use Latex to make the comment look appealing )

Jason Gomez - 6 months, 1 week ago

This method is applied incorrectly and happened to be the right answer by sheer luck, this does not generalize to the limit of the product of any 2 functions.

The missing rule here is that the limit can only be distributed like this to the product of 2 functions iff the limit of each of those parts exist. Here the second part which is the ln function does not have a limiting value

Shaumik Khanna - 2 months ago

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