Sine is not a sin cos cos is not a cause

Geometry Level 2

sin ( cos x ) OR cos ( sin x ) \sin(\cos x)\qquad \text{OR} \qquad \cos(\sin x)

For all real x x , which of these expressions is greater?

cos ( sin x ) \cos(\sin x) Both are always equal in value sin ( cos x ) \sin(\cos x)

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6 solutions

Otto Bretscher
Aug 27, 2015

We have cos ( sin x ) > sin ( cos x ) \cos(\sin{x})>\sin(\cos{x}) for all x x , based on the following five observations.

  1. cos ( sin x ) > cos ( x ) > sin ( cos x ) \cos(\sin{x})>\cos(x)>\sin(\cos{x}) on the interval ( 0 , π / 2 ) (0,\pi/2) ,
  2. cos ( sin x ) > 0 > sin ( cos x ) \cos(\sin{x})>0>\sin(\cos{x}) on the interval ( π / 2 , π ] (\pi/2,\pi] ,
  3. The inequality holds for x = 0 x=0 and x = π / 2 x=\pi/2 ,
  4. Both functions have a period of 2 π 2\pi , and
  5. Both functions are even.

So , you are back. Welcome! :)

Nihar Mahajan - 5 years, 9 months ago

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Not for long, though; just having a little fun between semesters...

Otto Bretscher - 5 years, 9 months ago
Pulkit Gupta
Dec 17, 2015

One way to solve the problem is to realize that while sin ( cos x ) \sin(\cos x) may attain negative values for some x , cos ( sin x ) \cos(\sin x) will always be a positive number. Hence, the inference.

P.S. Evidently this is a pretty anomalous solution, but given the limited scope of options it makes solving pretty fast.

Md Zuhair
May 31, 2017

sin x + cos x 2 \sin x+\cos x \leq \sqrt{2}

And 2 < π 2 \sqrt{2} < \dfrac{\pi}{2}

So

sin x + cos x 2 < π 2 \sin x+\cos x \leq \sqrt{2} < \dfrac{\pi}{2}

sin x < π 2 cos x \implies \sin x < \dfrac{\pi}{2} - \cos x

sin ( cos x ) < cos ( sin x ) \implies \sin( \cos x) < \cos ( \sin x)

Francis Derit
Aug 27, 2015

If x = 0 then, = sin (cos 0) = sin(1) = 0.84

=cos(sin 0) = cos(0) = 1

0.84 < 1 Ans. cos (sin x)

Aditya Pappula
Aug 27, 2015

This is valid if sinx or cosx is taken in degrees and not radians.

It is valid even in radians.

Ivan Koswara - 5 years, 9 months ago
Laxmi Ships
Aug 25, 2015

f(x)=sin(cos x) and g(x)=cos(sin x) so the images of f will variate between sin[0,1] which is [sin(0),sin(1)] = [0, 0.017] And the images of g will variate between cos[0,1] which is [0.999 ,1] therefore g(x)>f(x)

Copy-pasta. -_-

Azher Ferrer - 5 years, 9 months ago

First, f f between [ sin 1 , sin 1 ] [\sin -1, \sin 1] since cos \cos attains the value 1 -1 as well; similar with g g . Second sin 1 0.8414 , cos 1 0.5403 \sin 1 \approx 0.8414, \cos 1 \approx 0.5403 , so the solution is faulty (since sin 1 > cos 1 \sin 1 > \cos 1 and thus the two images intersect).

Ivan Koswara - 5 years, 9 months ago

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