Infinite L'Hôpital's Rule!

Calculus Level 1

Please help me! I kept using L'Hôpital's rule millions of times and I can't evaluate the limit below!

lim x 0 cot x csc x = ? \displaystyle \lim_{x \to 0} \frac {\cot x }{\csc x } = \, ?

Details and Assumptions:

  • d d x ( cot x ) = csc 2 x \frac{d}{dx}( \cot x ) = -\csc^2 x
  • d d x ( csc x ) = csc x cot x \frac{d}{dx}( \csc x ) = -\csc x \cot x


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

16 solutions

Trevor B.
May 10, 2014

lim x 0 cot ( x ) csc ( x ) = lim x 0 cos ( x ) sin ( x ) 1 sin ( x ) = lim x 0 cos ( x ) 1 = cos ( 0 ) = 1 \displaystyle\lim_{x\rightarrow0}\dfrac{\cot(x)}{\csc(x)}=\displaystyle\lim_{x\rightarrow0}\dfrac{\frac{\cos(x)}{\sin(x)}}{\frac{1}{\sin(x)}}=\displaystyle\lim_{x\rightarrow0}\dfrac{\cos(x)}{1}=\cos(0)=\boxed{1}

yes, same with me. no need l'hopital rule :)

Yudi Prabudi - 7 years, 1 month ago

@Pi Han Goh Congratulations on reaching 1000 followers! You deserve it, and keep going for the next thousand!

Trevor B. - 7 years, 1 month ago

Log in to reply

43242 43242

Pi Han Goh - 7 years, 1 month ago

Log in to reply

Yes welcome to the club @Pi Han Goh ! :D

Finn Hulse - 7 years, 1 month ago

Neat Solution using simple trig functions

Mohummud Husain - 6 years ago

same as i did

Vighnesh Raut - 7 years ago

I don't even know what L'Hospital's Rule, but I used Trevor's method.

Sharky Kesa - 7 years, 1 month ago

Log in to reply

@Sharky Kesa If you know derivatives for calculus, if you have an indeterminate form in the limit of a fraction f ( x ) g ( x ) \dfrac{f(x)}{g(x)} at a number k k that results in \dfrac{\infty}{\infty} or 0 0 , \dfrac{0}{0}, then lim x k f ( x ) g ( x ) = lim x k f ( x ) g ( x ) \displaystyle\lim_{x\rightarrow k}\frac{f(x)}{g(x)}=\displaystyle\lim_{x\rightarrow k}\frac{f'(x)}{g'(x)}

Trevor B. - 7 years, 1 month ago
Ishan Singh
May 10, 2014

If one insists on using L'Hôpital's Rule,

Let L = lim x 0 cot ( x ) csc ( x ) = lim x 0 csc ( x ) cot ( x ) \text{L}= \lim\limits_{x \to 0} \frac{\cot(x)}{\csc(x)} = \lim\limits_{x \to 0}\frac{\csc(x)}{\cot(x)} (using L' Hospital Rule)

L = 1 L \implies \text{L}=\dfrac{1}{\text{L}}

L = ± 1 \implies \text{L}= \pm 1
L = 1 \implies \boxed{\text{L}=1} ( 1 is rejected since the ratio cot ( x ) csc ( x ) is positive near 0 ) \left( \ -1 \ \text{is rejected since the ratio} \ \ \dfrac{\cot(x)}{\csc(x)} \ \ \text{is positive near} \ \ 0 \ \ \right)

That's innovative!

Aran Pasupathy - 6 years ago

It's L^2=1 and L=+1 or -1 If L^2=-1, then L becomes imaginary.

Srinivasakumar P - 5 years, 10 months ago

same as you

Adarsh singh - 6 years, 12 months ago

I had the same solution but that does not yet quality as a proof because it is unvertain if the limit exists. (Of course question implies it does.)

devilchn X - 4 years, 11 months ago

Why to do la hospital

Kumar Krish - 2 years ago
Samarth Sangam
Sep 8, 2014

lim x 0 cot x csc x = lim x 0 cos x sin x 1 sin x = lim x 0 cos x = 1 \lim _{ x\rightarrow 0 }{ \frac { \cot { x } }{ \csc { x } } } \\ =\lim _{ x\rightarrow 0 }{ \frac { \frac { \cos { x } }{ \sin { x } } }{ \frac { 1 }{ \sin { x } } } } \\ =\lim _{ x\rightarrow 0 }{ \cos { x } } \\ =1

Pratik Goel
May 10, 2014

No need to apply L-Hospital's. Simplify the expression by converting into sine and cosine to get l i m x 0 c o s x lim_{x→0}cosx to get answer 1

Moniqua Brown
Aug 14, 2015

I put absolutley no thought into this and just typed 1 and it was correct

Pulkit Gopalani
Jun 4, 2014

lim x 0 cot ( x ) csc ( x ) = lim x 0 cos ( x ) sin ( x ) ÷ 1 sin ( x ) = lim x 0 cos ( x ) = cos ( 0 ) = 1 \lim _{ x\rightarrow 0 }{ \frac { \cot { (x) } }{ \csc { (x) } } } =\quad \lim _{ x\rightarrow 0 }{ \frac { \cos { (x) } }{ \sin { (x) } } } \div \frac { 1 }{ \sin { (x) } } \quad =\quad \lim _{ x\rightarrow 0 }{ \cos { (x) } } =\quad \cos { (0)\quad } =\quad 1

Piyush Parwani
May 30, 2015

also done the same

Gaurav Kakked
May 29, 2015

ha ha ha very funny ..:) "Please help me! I kept using L'Hôpital's rule millions of times and I can't evaluate the limit!" love your humor.. cot x / cosec x = cosx...lim x>0 cos x = 1

Abby Salvadore
Jul 27, 2014

just simplify the trig ratio to be cos(x) , then the value of cos(0) is 1

Why are you keep on doing the lazy solution. this is actually the solution. lol hahaha

cot(x) = cos(x)/sin(x) csc(x) = sin (x)

cot(x)/csc(x) = cos(x)

limit as x->0 is 1. lol

Figel Ilham
Jun 26, 2014

I think you do not need to use L'Hopital rule

You just evaluate by manipulating it: limit(((cot(x))/(csc(x)))= limit(((((cos(x))/(sin(x))))/(((1)/(sin(x)))))= limit(((cos(x))/(sin(x)))·sin(x)= limit(cos(x)=1,x,0),x,0),x,0),x,0)

Ravi Meena
Jun 13, 2014

cot(x)/csc(x) = sin(x)/tan(x) now use L' Hospital's rule

Mohit Kuri
Jun 5, 2014

Cotx= cosx/sinx ; cosecx = 1/sinx ; thus cosx/cosecx=cosx as x-> 0 means x is not equal to 0 . Thus lim(x->0)cosx = 1

Ahmad Mustafa
Jun 4, 2014

You can convert cot and cosec into sin and cos then only cos(x) will remain and cos (0) is 1

Pretty neat explanation

Mohummud Husain - 6 years ago
Arvind Chander
May 10, 2014

Multiply and divide the given limit with x. And then use the taylor expansion for x cot(x) and x cosec (x) and then substitute x=0 to get the answer, that is 1

a round of applause to this guy^

Connor Kenway - 7 years, 1 month ago

The solution was a joke...the problem was so easy that I wanted to give a little complex soln!!

Arvind Chander - 7 years, 1 month ago
Taisanul Haque
Oct 31, 2018

If you wanted to solve the above limit problem by only using L'Hospital rule,You have to little bit manipulate the function as follows

Now apply L'Hospital rule

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...