is a square with side length 1. divides side into two equal parts. Circle centered at point is tangential to sides , and segment .
If is the point where circle touches , find the area of quadrilateral .
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Extend C D and B E to meet at H . Then we note circle k is the incircle of △ H B C with inradius r = F G = F I = F J . We note that △ A B E and △ H B C are similar. Then if ∠ A B E = θ , then ∠ I H F = 2 θ .
tan ∠ I H F ⟹ tan 2 θ = H I I F = H D + C D − C I r = A B + 1 − F J r = 1 + 1 − r r = 2 − r r Note that △ H D E and △ A B E are congruent.
We also note that tan θ = tan ∠ A B E = A B A E = 2 1 , then we have:
tan θ 1 − tan 2 2 θ 2 tan 2 θ tan 2 2 θ + 4 tan 2 θ − 1 ( 2 − r r ) 2 + 2 − r 4 r − 1 r 2 − 3 r + 1 ⟹ r = 2 1 = 2 1 = 0 = 0 = 0 = 2 3 − 5 Putting tan 2 θ = 2 − r r
Note that quadrilateral E G F D is made up of △ D E F and △ E F G , then:
[ E G F D ] = [ D E F ] + [ E F G ] = 2 D I × D E + 2 F G × E G = 2 ( 1 − r ) × 2 1 + 2 r ( 2 − 5 / 2 − r ) = 4 1 + ( 3 − 5 ) r − 2 r 2 = 4 1 + 2 r 2 − 2 r 2 = 4 1 = 0 . 2 5 See note. Note that r = 2 3 − 5
Note:
E G = H G − H E = H I − B E = H D + D I − 2 5 = A B + 1 − r − 2 5 = 1 + 1 − r − 2 5 = 2 − 2 5 − r Note that △ H F G and △ H F I are congruent. △ H D E and △ A B E are congruent.