Is there a LIMIT to the amount of limits problems?

Calculus Level 2

Calculate lim x π 2 ( x tan x π 2 tan x ) \large \lim\limits_{x \to \frac{\pi}{2}} \left( x\tan x - \frac{\pi}{2}\tan x\right)


The answer is -1.

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2 solutions

Chew-Seong Cheong
Aug 15, 2020

L = lim x π 2 ( x tan x π 2 tan x ) = lim x π 2 ( x π 2 ) tan x = lim x π 2 x π 2 cot x A 0/0 case, L’H o ˆ pital’s rule applies = lim x π 2 1 csc 2 x Differentiate up and down w.r.t. x . = 1 \begin{aligned} L & = \lim_{x \to \frac \pi 2} \left(x\tan x - \frac \pi 2 \tan x\right) \\ & = \lim_{x \to \frac \pi 2} \left(x-\frac \pi 2\right) \tan x \\ & = \lim_{x \to \frac \pi 2} \frac {x-\frac \pi 2}{\cot x} & \small \blue{\text{A 0/0 case, L'Hôpital's rule applies}} \\ & = \lim_{x \to \frac \pi 2} \frac 1{-\csc^2 x} & \small \blue{\text{Differentiate up and down w.r.t. }x.} \\ & = \boxed{-1} \end{aligned}


Reference: L'Hôpital's rule

With out L 'Hôpital's rule

Y = x tan ( x ) ( π / 2 ) tan ( x ) Y= x \tan (x) - (π/2) \tan (x)

= ( x π / 2 ) tan ( x ) =(x - π/2) \tan (x)

= ( x π / 2 ) cot ( π / 2 x ) =(x - π /2) \cot (π /2 - x)

= [ ( x π / 2 ) × cos ( π / 2 ) ] / ( sin ( π / 2 x ) ) =[(x - π /2) × \cos (π /2) ]/(\sin (π /2 - x))

As x x reaches to ( π / 2 ) (π /2)
sin ( π / 2 x ) = ( π / 2 x ) \sin(π /2-x)=(π /2 - x)

So the Y becomes ( x π / 2 ) / ( π / 2 x ) × cos ( π / 2 x ) (x-π /2)/(π /2-x) ×\cos (π /2 - x)

= 1 × cos ( z ) =-1×\cos (z) (where z z reaches to zero)

= 1 × 1 = 1 -1×1 =-1

how come when x x reaches π 2 \frac{\pi}{2} , sin ( π 2 x ) = π 2 x \sin \left(\frac{\pi}{2}-x \right) = \frac{\pi}{2}-x ?

James Watson - 7 months, 1 week ago

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s i n ( π / 2 x ) sin (π/2- x) = ( π / 2 x ) (π/2-x) (here (π/2-x ) reaches to zero)

For small angles. S i n ( z ) = z Sin(z )=z (here z reaches to 0)

Dwaipayan Shikari - 7 months, 1 week ago

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