Calculus is very easy

Calculus Level 3

lim x π / 2 1 + cos 2 x ( π 2 x ) 2 = ? \large \lim_{x\to \pi/2} \dfrac{1 + \cos2x}{(\pi - 2x)^2} = \, ?


The answer is 0.5.

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.

4 solutions

lim x π 2 1 + cos 2 x ( π 2 x ) 2 S i n c e i t i s a 0 0 f o r m , s o w e n e e d t o s i m p l i f y i t . lim x π 2 1 cos ( π 2 x ) ( π 2 x ) 2 lim x π 2 1 ( 1 2 sin 2 ( π 2 x ) 2 ) ( π 2 x ) 2 { U s i n g cos x = 1 2 sin 2 x 2 ) O p e n i n g t h e b r a c k e t s , lim x π 2 2 sin 2 ( π 2 x ) 2 ( π 2 x ) 2 lim x π 2 sin ( ( π 2 x ) 2 ) 2 ( π 2 x ) 2 2 M u l t i p l y i n g a n d d i v i d i n g 2 i n t h e d e n o m i n a t o r , lim x π 2 sin ( ( π 2 x ) 2 ) 2 ( ( π 2 x ) 2 ) 2 2 S e p a r a t i n g t e r m s , lim x π 2 sin ( ( π 2 x ) 2 ) 2 ( ( π 2 x ) 2 ) 2 × lim x π 2 1 2 B u t i f x π 2 , S o , 2 x π 0 , O r , π 2 x 0 C h a n g i n g l i m i t s lim π 2 x 0 sin ( ( π 2 x ) 2 ) 2 ( ( π 2 x ) 2 ) 2 × lim x π 2 1 2 B u t lim x 0 sin x x = 1 , S o , f i r s t t e r m = 1 T h e r e f o r e , lim x π 2 1 2 = 1 2 = 0.5 \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1+\cos { 2x } }{ { (\pi -2x) }^{ 2 } } } \\ Since\quad it\quad is\quad a\quad \frac { 0 }{ 0 } \quad form,\quad so\quad we\quad need\quad to\quad simplify\quad it.\\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1-\cos { (\pi -2x) } }{ { (\pi -2x) }^{ 2 } } } \\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1-(1-2\sin ^{ 2 }{ \frac { (\pi -2x) }{ 2 } } ) }{ { (\pi -2x) }^{ 2 } } } \quad \quad \quad \quad \quad \quad \{ Using\quad \cos { x } =\quad 1-2\sin ^{ 2 }{ \frac { x }{ 2 } } )\\ Opening\quad the\quad brackets,\\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 2\sin ^{ 2 }{ \frac { (\pi -2x) }{ 2 } } }{ { (\pi -2x) }^{ 2 } } } \\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { \sin { { (\frac { (\pi -2x) }{ 2 } })^{ 2 } } }{ { \frac { { (\pi -2x) }^{ 2 } }{ 2 } } } } \\ Multiplying\quad and\quad dividing\quad 2\quad in\quad the\quad denominator,\\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { \sin { { (\frac { (\pi -2x) }{ 2 } })^{ 2 } } }{ { { (\frac { (\pi -2x) }{ 2 } ) }^{ 2 }\quad }2 } } \\ Separating\quad terms,\\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { \sin { { (\frac { (\pi -2x) }{ 2 } })^{ 2 } } }{ { { (\frac { (\pi -2x) }{ 2 } ) }^{ 2 }\quad } } } \quad \times \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1 }{ 2 } } \\ But\quad if\quad x\rightarrow \frac { \pi }{ 2 } ,\quad So,\quad 2x-\pi \rightarrow 0,\quad Or,\quad \pi -2x\rightarrow 0\\ Changing\quad limits\\ \Rightarrow \quad \lim _{ \pi -2x\rightarrow 0 }{ \frac { \sin { { (\frac { (\pi -2x) }{ 2 } })^{ 2 } } }{ { { (\frac { (\pi -2x) }{ 2 } ) }^{ 2 }\quad } } } \times \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1 }{ 2 } } \\ But\quad \lim _{ x\rightarrow 0 }{ \frac { \sin { x } }{ x } } =1,\quad So,\quad first\quad term\quad =\quad 1\\ Therefore,\\ \Rightarrow \quad \lim _{ x\rightarrow \frac { \pi }{ 2 } }{ \frac { 1 }{ 2 } } \quad =\quad \frac { 1 }{ 2 } \quad =\quad 0.5

Otto Bretscher
Mar 13, 2016

Substituting t = π 2 x t=\frac{\pi}{2}-x we have lim t 0 1 + cos ( π 2 t ) 4 t 2 = lim t 0 1 cos ( 2 t ) 4 t 2 = lim t 0 2 sin 2 ( t ) 4 t 2 = 0.5 \lim_{t\to 0}\frac{1+\cos(\pi-2t)}{4t^2}=\lim_{t\to 0}\frac{1-\cos(2t)}{4t^2}=\lim_{t\to 0}\frac{2\sin^2(t)}{4t^2}=\boxed{0.5}

Moderator note:

Due to our familiarity with Maclaurin series, it is often helpful to use a change of variables so that we're evaluating the limit at 0.

Applying Taylor series of cos x \cos x around x = π x = \pi , then cos x = 1 + ( x π ) 2 2 ! . . . . \cos x = -1 + \frac{(x - \pi)^2}{2!} - .... lim x π 2 1 + cos 2 x ( π 2 x ) 2 = lim x π 2 ( 2 x π ) 2 2 ! ( π 2 x ) 2 = 0.5 \lim_{x \to \frac{\pi}{2}} \frac{1 + \cos 2x}{(\pi - 2x)^2} = \lim_{x \to \frac{\pi}{2}} \frac{\frac{(2x - \pi)^2}{2!}}{(\pi - 2x)^2} = 0.5

Alexandre Serrano
Mar 21, 2015

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...