In the diagram above .
Find the area of the shaded quadrilateral above.
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I filled in the angles in the diagram above using properties of an equilateral triangle, isosceles triangle and the measure of a straight angle.
Let m ∠ F B C = θ , m ∠ F C B = λ , F E = a and B E = x .
Using △ D B C ⟹ λ = 6 0 − θ
Using the law of sines on △ F B E ⟹ x = sin ( θ ) a
Using the law of sines on △ F E C ⟹ 2 = sin ( 6 0 − θ ) a ⟹ a = 2 sin ( 6 0 − θ ) ⟹
x = sin ( θ ) 2 ( sin ( 6 0 ∘ ) cos ( θ ) − cos ( 6 0 ∘ ) sin ( θ ) ) = 3 cot ( θ ) − 1 .
Using △ F B E ⟹ cot ( θ ) = a 1 and x 2 = a 2 + 1 ⟹ a = x 2 − 1 ⟹
cot ( θ ) = x 2 − 1 1 ⟹ x = x 2 − 1 3 − 1 ⟹ ( x + 1 ) x 2 − 1 = 3
⟹ ( x 2 + 2 x + 1 ) ( x 2 − 1 ) = 3 ⟹ x 4 + 2 x 3 − 2 x − 4 = 0
⟹ x 3 ( x + 2 ) − 2 ( x + 2 ) = 0 ⟹ ( x 3 − 2 ) ( x + 2 ) = 0
x > 0 ⟹ x = − 2 ⟹ x = 2 3 1 ⟹ a = 2 3 2 − 1 .
Let F C = l
m ∠ B E F = 9 0 − θ ⟹ m ∠ F E C = 9 0 + θ and sin ( θ ) = x a ⟹
Using the law of cosines on △ F E C ⟹ l 2 = a 2 + 1 − 2 a cos ( 9 0 + θ ) =
a 2 + 1 + 2 a sin ( θ ) = a 2 + 1 + x 2 a 2 = 2 3 2 − 1 + 1 + ( 2 3 2 ) ( 2 3 2 − 1 ) =
2 3 2 ( 1 + 2 3 2 − 1 ) = 2 3 4 ⟹ l = 2 3 2
⟹ A △ F E C = 2 1 2 3 2 ( a sin ( 3 0 ∘ ) = 2 1 2 3 2 2 3 2 − 1 ( 2 1 ) = 2 3 4 2 3 2 − 1
A △ F B E = 2 1 ( a ) ( 1 ) = 2 2 3 2 − 1
A △ F D B = 2 1 ( 1 ) ( 2 3 ) = 4 3
Let A F = m
Using the law of cosines on △ A D F ⟹
m 2 = 2 − 2 cos ( 1 2 0 ∘ ) = 2 + 2 ( 2 1 ) = 3 ⟹ m = 3 ⟹
A △ A D F = 2 1 3 ( sin ( 3 0 ∘ ) = 4 3
⟹ A = 2 3 4 2 3 2 − 1 + 2 2 3 2 − 1 + 2 3 ≈ 1 . 5 5 3 3 9 0 2 2