sin ( cos x ) \sin (\cos x) vs. cos ( sin x ) \cos (\sin x)

Geometry Level 2

If 0 < x < π 2 0<x<\frac π2 , then which is greater?

A = sin ( cos x ) \textbf A = \sin (\cos x)

B = cos ( sin x ) \textbf B = \cos (\sin x)

B \textbf B for 0 < x < π 4 0<x<\frac π4 A \textbf A B \textbf B A \textbf A for 0 < x < π 4 0<x<\frac π4

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4 solutions

sin x + cos x 2 < π 2 \sin x+\cos x\leq \sqrt 2 <\frac{π}{2} (since 3 < π < 4 3<π<4 and 8 < 9 2 < 1.5 8<9\implies \sqrt 2 <1.5 ), therefore cos x < π 2 sin x sin ( cos x ) < sin ( π 2 sin x ) = cos ( sin x ) \cos x<\frac{π}{2}-\sin x\implies \boxed {\sin (\cos x) <\sin (\frac{π}{2}-\sin x) =\cos (\sin x)}

Adhiraj Dutta
May 27, 2020

Desmos Desmos

Chew-Seong Cheong
May 25, 2020

B = cos ( sin x ) = sin ( π 2 sin x ) B = \cos (\sin x) = \sin \left(\dfrac \pi 2 - \sin x \right) . Since sin x + cos x = 2 sin ( x + π 4 ) 2 < π 2 \sin x + \cos x = \sqrt 2 \sin \left(x + \dfrac \pi 4\right) \le \sqrt 2 < \dfrac \pi 2 . Then π 2 sin x > cos x \dfrac \pi 2 - \sin x > \cos x . Therefore, B = sin ( π 2 sin x ) > sin ( cos x ) = A B = \sin \left(\dfrac \pi 2 - \sin x \right) > \sin (\cos x) = A or B > A \boxed B > A .

Sir,please check my solution

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I'm not sure. At first I was also trying this approach.

Chew-Seong Cheong - 1 year ago

Ok sir,thanks for paying attention

In the folllowing range we can,say that sinx<x......(1) ..so,cos(sinx)>cosx.....(2) Put...x=cosx in equation 1.. So,cosx>sin(cosx)......(3) Therefore, Cos(sinx)>sin(cosx)......... Please comment if you figure out some mistake

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