The floor is the limit!!

Calculus Level 2

Evaluate the following limit:

lim x 0 sin x × tan x x 2 \lim\limits_{x\to 0}\left \lfloor{\dfrac{\sin x\times \tan x}{x^{2}}}\right \rfloor

Note: . \left \lfloor{.}\right \rfloor denotes the greatest integer function .


The answer is 1.

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2 solutions

lim x 0 sin x tan x x 2 = lim x 0 sin 2 x x 2 cos x \displaystyle \lim_{x\to 0} {\left \lfloor \dfrac{\sin{x}\tan{x}}{x^2} \right \rfloor} = \lim_{x\to 0} {\left \lfloor \dfrac{\sin^2{x}}{x^2\cos{x}} \right \rfloor}

Using Maclaurin expansion, we have,

lim x 0 sin 2 x x 2 cos x = lim x 0 ( x x 3 3 ! + x 5 5 ! . . . ) 2 x 2 ( 1 x 2 2 ! + x 4 4 ! . . . ) = x 2 x 2 ( 1 ) = 1 \displaystyle \lim_{x\to 0} {\left \lfloor \dfrac{\sin^2{x}}{x^2\cos{x}} \right \rfloor} = \lim_{x\to 0} {\left \lfloor \dfrac{\left(x-\frac{x^3}{3!} + \frac{x^5}{5!}-... \right)^2}{x^2\left(1-\frac{x^2}{2!} + \frac{x^4}{4!}-... \right)} \right \rfloor} = \left \lfloor \dfrac{x^2}{x^2\left(1 \right)} \right \rfloor = \boxed{1}

Edward Gelernt
Oct 8, 2014

lim x 0 sin x × tan x x 2 = lim x 0 sin x × sin x cos x x 2 = lim x 0 sin 2 x cos x × x 2 = lim x 0 sin 2 x x 2 × lim x 0 1 cos x = lim x 0 sin x x 2 × lim x 0 1 cos x W e k n o w t h a t lim x 0 sin x x = 1 , s o c o n t i n u i n g o n : = 1 2 × lim x 0 1 cos x = 1 cos ( 0 ) = 1 1 = 1 \lim _{ x\rightarrow 0 }{ \left\lfloor \frac { \sin { x } \times \tan { x } }{ { x }^{ 2 } } \right\rfloor } \\ =\lim _{ x\rightarrow 0 }{ \left\lfloor \frac { \sin { x } \times \frac { \sin { x } }{ \cos { x } } }{ { x }^{ 2 } } \right\rfloor } \\ =\lim _{ x\rightarrow 0 }{ \left\lfloor \frac { \sin ^{ 2 }{ x } }{ \cos { x } \times { x }^{ 2 } } \right\rfloor } \\ =\lim _{ x\rightarrow 0 }{ \left\lfloor \frac { \sin ^{ 2 }{ x } }{ \quad { x }^{ 2 } } \right\rfloor } \times \lim _{ x\rightarrow 0 } \left\lfloor \frac { 1 }{ \cos { x } } \right\rfloor \\ =\lim _{ x\rightarrow 0 }{ { \left\lfloor \frac { \sin { x } }{ x } \right\rfloor }^{ 2 } } \times \lim _{ x\rightarrow 0 } \left\lfloor \frac { 1 }{ \cos { x } } \right\rfloor \\ We\quad know\quad that\quad \lim _{ x\rightarrow 0 }{ { \frac { \sin { x } }{ x } } } =\quad 1,\quad so\quad continuing\quad on:\\ ={ \left\lfloor 1 \right\rfloor }^{ 2\quad }\times \lim _{ x\rightarrow 0 } \left\lfloor \frac { 1 }{ \cos { x } } \right\rfloor \\ =\left\lfloor \frac { 1 }{ \cos { (0) } } \right\rfloor \\ =\left\lfloor \frac { 1 }{ 1 } \right\rfloor \\ =1

can you please explain step 4 to step 5

Sarthak Nandan - 6 years, 7 months ago

Is Limit distributive here? I have problem in this step ( step 4)

Prakash Chandra Rai - 6 years, 2 months ago

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