find x in triangle

Geometry Level 3


The answer is 40.

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3 solutions

Drop a perpendicular from A A to B C BC at E E . Let A B = C D = 1 AB=CD=1 . Then A E = sin 2 0 |AE|=\sin 20^\circ , also

D E + E C = D C A E tan A D C + A E tan A C D = 1 sin 2 0 tan 3 0 + sin 2 0 tan x = 1 \begin{aligned} |DE|+|EC| & = |DC| \\ \frac {|AE|}{\tan \angle ADC} + \frac {|AE|}{\tan \angle ACD} & = 1 \\ \frac {\sin 20^\circ}{\tan 30^\circ} + \frac {\sin 20^\circ}{\tan x^\circ} & = 1 \end{aligned}

1 tan x = 1 sin 2 0 1 tan 3 0 = 1 sin 2 0 cos 3 0 sin 3 0 = sin 3 0 sin 2 0 cos 3 0 sin 2 0 sin 3 0 = sin 3 0 cos 7 0 cos 3 0 sin 2 0 sin 3 0 = 1 2 1 2 ( cos 4 0 + cos 10 0 ) 1 2 ( cos 1 0 cos 5 0 ) = 1 cos 4 0 + cos 8 0 sin 8 0 sin 4 0 = 1 cos 4 0 + 2 cos 2 4 0 1 2 sin 4 0 cos 4 0 sin 4 0 = cos 4 0 ( 2 cos 4 0 1 ) sin 4 0 ( 2 cos 4 0 1 ) x = 40 \begin{aligned} \implies \frac 1{\tan x^\circ} & = \frac 1{\sin 20^\circ} - \frac 1{\tan 30^\circ} \\ & = \frac 1{\sin 20^\circ} - \frac {\cos 30^\circ}{\sin 30^\circ} \\ & = \frac {\sin 30^\circ - \blue{\sin 20^\circ} \cos 30^\circ}{\sin 20^\circ \sin 30^\circ} \\ & = \frac {\sin 30^\circ - \blue{\cos 70^\circ}\cos 30^\circ}{\sin 20^\circ \sin 30^\circ} \\ & = \frac {\frac 12 - \frac 12 (\cos 40^\circ + \cos 100^\circ)}{\frac 12 (\cos 10^\circ - \cos 50^\circ)} \\ & = \frac {1 - \cos 40^\circ + \cos 80^\circ}{\sin 80^\circ - \sin 40^\circ} \\ & = \frac {1 - \cos 40^\circ + 2\cos^2 40^\circ - 1}{2\sin 40^\circ \cos 40^\circ - \sin 40^\circ} \\ & = \frac {\cos 40^\circ \cancel{(2\cos 40^\circ - 1)}}{\sin 40^\circ \cancel{(2\cos 40^\circ - 1)}} \\ \implies x & = \boxed{40} \end{aligned}

nice solution sir!

nibedan mukherjee - 1 year, 1 month ago

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Glad that you like it.

Chew-Seong Cheong - 1 year, 1 month ago
Trupal Panchal
May 7, 2020

take a compass put it on the screen at the angle 'x' you have your answer 40. i know it seems idiotic but try it, no use of math only a tool called compass and boom you got your answer.

"smart people never do hard works, and i do it smarty." -trupal panchal

tan x = 2 sin 20 ° sin 30 ° 1 2 sin 20 ° cos 30 ° = tan 40 ° x = 40 ° \tan x=\dfrac{2\sin 20\degree\sin 30\degree}{1-2\sin 20\degree\cos 30\degree}=\tan 40\degree\implies x=\boxed {40\degree} .

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