Do these limit problems have a limit?

Calculus Level 3

lim x 0 + ( 1 + sin ( 4 x ) ) cot ( x ) \large \lim_{x \to 0^+} (1+\sin(4x))^{\cot(x)}

Evaluate the limit above.

e 4 e^4 4 4 e 4^e 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.

3 solutions

Chew-Seong Cheong
Dec 19, 2018

Relevant wiki: L'Hopital's Rule - Basic

L = lim x 0 + ( 1 + sin ( 4 x ) ) cot x A 1 case, the following applies = exp ( lim x 0 + cot x sin ( 4 x ) ) lim x a ( f ( x ) ) h ( x ) = exp ( lim x a h ( x ) ( f ( x ) 1 ) ) = exp ( lim x 0 + sin ( 4 x ) tan x ) A 0/0 case, L’H o ˆ pital’s rule applies = exp ( lim x 0 + 4 cos ( 4 x ) sec 2 x ) Differentiate up and down w.r.t. x = e 4 \begin{aligned} L & = \lim_{x \to 0^+} (1+\sin (4x))^{\cot x} & \small \color{#3D99F6} \text{A }1^\infty \text{ case, the following applies} \\ & = \exp \left(\lim_{x \to 0^+} \cot x \sin (4x)\right) & \small \color{#3D99F6} \lim_{x \to a} (f(x))^{h(x)} = \exp \left(\lim_{x \to a} h(x)(f(x)-1) \right) \\ & = \exp \left(\color{#3D99F6} \lim_{x \to 0^+} \frac {\sin (4x)}{\tan x} \right) & \small \color{#3D99F6} \text{A 0/0 case, L'Hôpital's rule applies} \\ & = \exp \left(\color{#3D99F6} \lim_{x \to 0^+} \frac {4 \cos (4x)}{\sec^2 x} \right) & \small \color{#3D99F6} \text{Differentiate up and down w.r.t. }x \\ & = \boxed {e^4} \end{aligned}


Reference : 1 \color{#3D99F6}1^\infty limit (see: 2nd method )

Parth Sankhe
Dec 19, 2018

lim f ( x ) , g ( x ) 1 , ( f ( x ) ) g ( x ) = e lim ( f ( x ) 1 ) ( g ( x ) ) \displaystyle \lim _{f(x),g(x)\rightarrow 1,∞} (f(x))^{g(x)}=e^{\lim (f(x)-1)(g(x))}

Using the same formula, we get the limit as e sin 4 x tan x = e 4 e^{\frac {\sin 4x}{\tan x}}=e^4

Amal Hari
Dec 18, 2018

Let y = ( 1 + s i n ( 4 x ) ) c o t ( x ) y=(1+sin(4x))^{cot(x)}

Since direct substitution results in an indeterminate form 1 i n f 1^{inf}

we can take natural log of this equation

l n ( y ) = c o t ( x ) l n ( 1 + s i n ( 4 x ) ) ln(y)=cot(x) *ln(1+sin(4x))

l n ( y ) = l n ( 1 + s i n ( 4 x ) ) / ( 1 / c o t ( x ) ) ln(y)=ln(1+sin(4x))/(1/cot(x))

l n ( y ) = l n ( 1 + 4 s i n ( x ) ) / t a n ( x ) ln(y)=ln(1+4sin(x))/tan(x)

Since the answer is still indeterminate as 0/0

Apply L'Hospitals rule

4 c o s ( x ) / ( 1 + 4 s i n x ) s e c 2 x 4cos(x)/(1+4sinx)*sec^2x

4 c o s 3 ( x ) / ( 1 + 4 s i n x ) 4cos^3(x)/(1+4sinx)

Substituting x=0

4/1

ln(y) when x approaches 0=4

*y=e^4 ,the limit of function. *

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...