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Calculus Level 3

lim x 0 sin x log ( 1 + tan x ) = ? \Large \lim_{x\rightarrow 0 } \dfrac{ \sin x}{\log(1 + \tan x)} = \ ?


The answer is 1.00.

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3 solutions

Naren Bhandari
Jun 12, 2018

lim x 0 sin x log ( 1 + tan x ) = lim x 0 sin x tan x log ( 1 + tan x ) tan x = lim x 0 cos x lim x 0 log ( 1 + tan x ) tan x = 1 \lim_{x\to 0} \dfrac{\sin x}{\log(1+\tan x)} = \lim_{x\to 0} \dfrac{\dfrac{\sin x}{\tan x}}{\dfrac{\log(1+\tan x)}{\tan x}} = \dfrac{\lim_{x \to 0} \cos x}{\lim_{x\to 0} \dfrac{\log (1+\tan x)}{\tan x }} =\boxed{1}


Relevant wiki : L-Hopital's Rule lim x 0 sin x log ( 1 + tan x ) = lim x 0 cos x sec 2 x 1 + tan x = lim x 0 cos x ( 1 + tan x ) sec 2 x = 1 \lim_{x\to 0} \dfrac{\sin x}{\log(1+\tan x)} = \lim_{x\to 0} \dfrac{\cos x}{\dfrac{\sec ^2x}{1+\tan x}}=\lim_{x\to 0}\dfrac{\cos x\,(1+\tan x)}{\sec ^2x}=\boxed{1}


Relevant wiki : Taylor Series We have sin x = x x 3 3 ! + x 5 5 ! x 7 7 ! + & log ( 1 + tan x ) = tan x tan 2 x 2 + tan 3 x 3 tan 4 x 4 + log ( 1 + tan x ) = x + x 3 3 + 2 x 5 15 + + ( x + x 3 3 + 2 x 5 15 + ) 2 2 + \sin x = x -\dfrac{x^3}{3!} + \dfrac{x^5}{5!} -\dfrac{x^7}{7!} +\cdots \\ \text{\&}\quad \log(1 +\tan x) = \tan x-\dfrac{\tan ^2x}{2} +\dfrac{\tan ^3x}{3} -\dfrac{\tan ^4x}{4}+\cdots \\ \log(1+\tan x) = x+\dfrac{x^3}{3} +\dfrac{2x^5}{15} +\cdots +\dfrac{\left(x+\dfrac{x^3}{3} +\dfrac{2x^5}{15} +\cdots\right) ^2}{2} + Now L = lim x 0 [ x x 3 3 ! + x 5 5 ! x 7 7 ! + x + x 3 3 + 2 x 5 15 + + ( x + x 3 3 + 2 x 5 15 + ) 2 2 + ] L = lim x 0 x x [ 1 x 2 3 ! + x 4 5 ! x 6 7 ! + 1 + x 2 3 + 2 x 4 15 + + x ( x + x 2 3 + 2 x 4 15 + ) 2 2 + ] L = 1 1 = 1 L = \lim_{x\to 0} \left[ \dfrac{x -\dfrac{x^3}{3!} + \dfrac{x^5}{5!} -\dfrac{x^7}{7!} +\cdots}{ x+\dfrac{x^3}{3} +\dfrac{2x^5}{15} +\cdots +\dfrac{\left(x+\dfrac{x^3}{3} +\dfrac{2x^5}{15} +\cdots\right) ^2}{2} +\cdots } \right]\\ L = \lim_{x\to 0}\dfrac{x}{x} \left[ \dfrac{1 -\dfrac{x^2}{3!} + \dfrac{x^4}{5!} -\dfrac{x^6}{7!} +\cdots}{ 1+\dfrac{x^2}{3} +\dfrac{2x^4}{15} +\cdots +\dfrac{ x\left(x+\dfrac{x^2}{3} +\dfrac{2x^4}{15} +\cdots\right) ^2}{2} +\cdots } \right] \\ L = \dfrac{1}{1} =\boxed{1}


@Naren Bhandari Great!!!...for better understanding of others you should mention the name of method used in each case.

Ravneet Singh - 2 years, 12 months ago

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Thank you !! I have added the wiki's for more info. :)

Naren Bhandari - 2 years, 12 months ago
Daniel G
Jun 17, 2018

At x=0, sin(x) and tan(x) are like x, and log(1+f(x)) is like f(x). So our limit is equivalent to the limit x/x, which is 1

Chew-Seong Cheong
Jun 13, 2018

Relevant wiki: L'Hopital's Rule - Basic

L = lim x 0 sin x log ( 1 + tan x ) A 0/0 case, L’H o ˆ pital’s rule applies. = lim x 0 cos x sec 2 x 1 + tan x Differentiate up and down w.r.t. x . = 1 \begin{aligned} L & = \lim_{x \to 0} \frac {\sin x}{\log (1+\tan x)} & \small \color{#3D99F6} \text{A 0/0 case, L'Hôpital's rule applies.} \\ & = \lim_{x \to 0} \frac {\cos x}{\frac {\sec^2 x}{1+\tan x}} & \small \color{#3D99F6} \text{Differentiate up and down w.r.t. }x. \\ & = \boxed{1}\end{aligned}

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