Find x in square

Geometry Level 3


The answer is 65.

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2 solutions

Trupal Panchal
May 7, 2020

consider triangle ADF and ABE (sorry to say i still don't know how to answer which look beautiful enough in the way of math) AD=BD cos(20) AF = cos(25) AE

A F A E \frac{AF}{AE} = c o s ( 25 ) c o s ( 20 ) \frac{cos(25)}{cos(20)} for triangle AEF take a perpendicular to EF which go though A. now we have two triangles AOE and AOF

so AO = sin(x) AE and AO = sin(y) AF

A F A E \frac{AF}{AE} = s i n ( x ) s i n ( y ) \frac{sin (x)}{sin(y)} = c o s ( 25 ) c o s ( 20 ) \frac{cos(25)}{cos(20)} = s i n ( 65 ) s i n ( 70 ) \frac{sin(65)}{sin(70)}

x y \frac{x}{y} = 65 70 \frac{65}{70}

x= 65k ( k is a constant) for triangle k=1

so x = 65

By angle chasing, we find that B A E = 2 5 \angle BAE = 25^\circ and A E B = 6 5 \angle AEB = 65^\circ . Then F E C = 18 0 6 5 x = 11 5 x \angle FEC = 180^\circ - 65^\circ - x^\circ = 115^\circ - x^\circ and

tan ( 11 5 x ) = F C C E = 1 tan 2 0 1 tan 2 5 Let A B = B C = C D = D A = 1 = 1 tan ( 4 5 2 0 ) 1 tan 2 5 = 1 1 tan 2 5 1 + tan 2 5 1 tan 2 5 = 2 tan 2 5 1 tan 2 2 5 = tan 5 0 11 5 x = 5 0 x = 65 \begin{aligned} \tan (115^\circ - x^\circ) & = \frac {FC}{CE} = \frac {1-\tan 20^\circ}{1-\tan 25^\circ} & \small \blue{\text{Let }AB=BC=CD=DA=1} \\ & = \frac {1-\tan (45^\circ - 20^\circ)}{1-\tan 25^\circ} = \frac {1-\frac {1-\tan 25^\circ}{1+\tan 25^\circ}}{1-\tan 25^\circ} \\ & = \frac {2\tan 25^\circ}{1-\tan^2 25^\circ} = \tan 50^\circ \\ 115^\circ - x^\circ & = 50^\circ \\ \implies x & = \boxed{65} \end{aligned}

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