Strange quadrilateral

Geometry Level 2

A B C D ABCD is a square with side length 1. E E divides side A D AD into two equal parts. Circle k k centered at point F F is tangential to sides B C BC , C D CD and segment B E BE .

If G G is the point where circle k k touches B E BE , find the area of quadrilateral E G F D EGFD .


The answer is 0.25.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Chew-Seong Cheong
Sep 20, 2017

Extend C D CD and B E BE to meet at H H . Then we note circle k k is the incircle of H B C \triangle HBC with inradius r = F G = F I = F J r=FG=FI=FJ . We note that A B E \triangle ABE and H B C \triangle HBC are similar. Then if A B E = θ \angle ABE = \theta , then I H F = θ 2 \angle IHF = \dfrac \theta 2 .

tan I H F = I F H I = r H D + C D C I Note that H D E and A B E are congruent. = r A B + 1 F J = r 1 + 1 r tan θ 2 = r 2 r \begin{aligned} \tan \angle IHF & = \frac {IF}{HI} \\ & = \frac {r}{{\color{#3D99F6}HD}+CD-\color{#D61F06}CI} & \small \color{#3D99F6} \text{Note that }\triangle HDE \text{ and }\triangle ABE \text{ are congruent.} \\ & = \frac {r}{{\color{#3D99F6}AB}+1-\color{#D61F06}FJ} \\ & = \frac {r}{{\color{#3D99F6}1}+1-\color{#D61F06}r} \\ \implies \tan \frac \theta 2 & = \frac r{2-r} \end{aligned}

We also note that tan θ = tan A B E = A E A B = 1 2 \tan \theta = \tan \angle ABE = \dfrac {AE}{AB} = \dfrac 12 , then we have:

tan θ = 1 2 2 tan θ 2 1 tan 2 θ 2 = 1 2 tan 2 θ 2 + 4 tan θ 2 1 = 0 Putting tan θ 2 = r 2 r ( r 2 r ) 2 + 4 r 2 r 1 = 0 r 2 3 r + 1 = 0 r = 3 5 2 \begin{aligned} \tan \theta & = \frac 12 \\ \frac {2\tan \frac \theta 2}{1-\tan^2 \frac \theta 2} & = \frac 12 \\ \tan^2 \frac \theta 2 + 4 \tan \frac \theta 2 - 1 & = 0 & \small \color{#3D99F6} \text{Putting }\tan \frac \theta 2 = \frac r{2-r} \\ \left(\frac r{2-r}\right)^2 + \frac {4r}{2-r} - 1 & = 0 \\ r^2 - 3r +1 & = 0 \\ \implies r & = \frac {3-\sqrt 5}2 \end{aligned}

Note that quadrilateral E G F D EGFD is made up of D E F \triangle DEF and E F G \triangle EFG , then:

[ E G F D ] = [ D E F ] + [ E F G ] = D I × D E 2 + F G × E G 2 = ( 1 r ) × 1 2 2 + r ( 2 5 / 2 r ) 2 See note. = 1 + ( 3 5 ) r 2 r 2 4 Note that r = 3 5 2 = 1 + 2 r 2 2 r 2 4 = 1 4 = 0.25 \begin{aligned} [EGFD] & = [DEF]+[EFG] \\ & = \frac {DI\times DE}2 + \frac {FG\times \color{#3D99F6} EG}2 \\ & = \frac {(1-r)\times \frac 12}2 + \frac {r {\color{#3D99F6}\left(2-\sqrt 5/2 - r\right)}}2 & \small \color{#3D99F6} \text{See note.} \\ & = \frac {1+(3-\sqrt 5)r - 2r^2}4 & \small \color{#3D99F6} \text{Note that } r = \frac {3-\sqrt 5}2 \\ & = \frac {1+2r^2-2r^2}4 \\ & = \frac 14 = \boxed{0.25} \end{aligned}


Note:

E G = H G H E Note that H F G and H F I are congruent. = H I B E H D E and A B E are congruent. = H D + D I 5 2 = A B + 1 r 5 2 = 1 + 1 r 5 2 = 2 5 2 r \begin{aligned} EG & = {\color{#3D99F6}HG} - {\color{#D61F06}HE} & \small \color{#3D99F6} \text{Note that }\triangle HFG \text{ and }\triangle HFI \text{ are congruent.} \\ & = {\color{#3D99F6}HI} - {\color{#D61F06}BE} & \small \color{#D61F06} \triangle HDE \text{ and }\triangle ABE \text{ are congruent.} \\ & = {\color{#D61F06}HD} + DI - {\color{#D61F06}\frac {\sqrt 5}2} \\ & = {\color{#D61F06}AB} +1-r - \frac {\sqrt 5}2 \\ & = {\color{#D61F06}1} +1-r - \frac {\sqrt 5}2 \\ & = 2 - \frac {\sqrt 5}2 - r \end{aligned}

No trigonometry:

r=(3-√5)/2

BG = BJ = 1- r = ½ (√5 – 1)

EG = BE – BG = = ½ √5 - ½ (√5 – 1) = ½

SDFGE = SDFE + SEFG = ½ ½ (1 – r) + ½ ½ r = ¼

Ilan Amity - 3 years, 8 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...