It's trigo time #7

Geometry Level 2

cos x ( 1 + tan 2 x ) csc x = 1 \large \frac {\cos x^\circ(1+\tan^2 x^\circ)}{\csc x^\circ} =1

In the domain [ 0 , 360 ] [ 0, 360 ] , what are the solutions of x x ?


This is a part of the set: It's trigo time

90 or 270 335 or 125 15 or 75 225 or 45 30 or 60 20 or 80

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2 solutions

Syed Hamza Khalid
May 13, 2017

So x x should equal to either 225 or 45 because:

t a n 45 = 1 tan\ 45=1 and t a n 225 = 1 tan\ 225=1

So our answer is

225 or 45

Chew-Seong Cheong
May 14, 2017

Relevant wiki: Half Angle Tangent Substitution

cos x ( 1 + tan 2 x ) csc x = 1 Note that csc x = 1 sin x sin x cos x ( 1 + tan 2 x ) = 1 And sin 2 x = 2 sin x cos x 1 2 sin 2 x ( 1 + tan 2 x ) = 1 Using half-angle tangent substitution 1 2 2 tan x 1 + tan 2 x ( 1 + tan 2 x ) = 1 tan x = 1 x = 4 5 or 22 5 \begin{aligned} \frac {\cos x(1+\tan^2 x)}{\csc x} & = 1 & \small \color{#3D99F6} \text{Note that } \csc x = \frac 1{\sin x} \\ \sin x \cos x(1+\tan^2 x) & = 1 & \small \color{#3D99F6} \text{And } \sin 2x = 2 \sin x \cos x \\ \frac 12 {\color{#3D99F6} \sin 2x} (1+\tan^2 x) & = 1 & \small \color{#3D99F6} \text{Using half-angle tangent substitution} \\ \frac 12 \cdot {\color{#3D99F6} \frac {2\tan x}{1+\tan^2 x}} \cdot (1+\tan^2 x) & = 1 \\ \tan x & = 1 \\ \implies x & = \boxed{45^\circ \text { or } 225^\circ} \end{aligned}

Nice solution sir.

Anmol Shetty - 4 years ago

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