Area Of A Quadrilateral!

Geometry Level pending

In the diagram above A D = D B = F B = D F = E C = 1 AD = DB = FB = DF = EC = 1 .

Find the area of the shaded quadrilateral A B C F ABCF above.


The answer is 1.55339022.

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1 solution

Rocco Dalto
Jan 28, 2020

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 m\angle{FBC} = \theta, \:\ m\angle{FCB} = \lambda, \:\ FE = a and B E = x BE = x .

Using D B C λ = 60 θ \triangle{DBC} \implies \lambda = 60 - \theta

Using the law of sines on F B E x = a sin ( θ ) \triangle{FBE} \implies x = \dfrac{a}{\sin(\theta)}

Using the law of sines on F E C 2 = a sin ( 60 θ ) a = 2 sin ( 60 θ ) \triangle{FEC} \implies 2 = \dfrac{a}{\sin(60 - \theta)} \implies a = 2\sin(60 - \theta) \implies

x = 2 ( sin ( 6 0 ) cos ( θ ) cos ( 6 0 ) sin ( θ ) ) sin ( θ ) x = \dfrac{2(\sin(60^{\circ})\cos(\theta) - \cos(60^{\circ})\sin(\theta))}{\sin(\theta)} = 3 cot ( θ ) 1 = \sqrt{3}\cot(\theta) - 1 .

Using F B E cot ( θ ) = 1 a \triangle{FBE} \implies \cot(\theta) = \dfrac{1}{a} and x 2 = a 2 + 1 a = x 2 1 x^2 = a^2 + 1 \implies a = \sqrt{x^2 - 1} \implies

cot ( θ ) = 1 x 2 1 x = 3 x 2 1 1 \cot(\theta) = \dfrac{1}{\sqrt{x^2 - 1}} \implies x = \dfrac{\sqrt{3}}{\sqrt{x^2 - 1}} - 1 \implies ( x + 1 ) x 2 1 = 3 (x + 1)\sqrt{x^2 - 1} = \sqrt{3}

( x 2 + 2 x + 1 ) ( x 2 1 ) = 3 x 4 + 2 x 3 2 x 4 = 0 \implies (x^2 + 2x + 1)(x^2 - 1) = 3 \implies x^4 + 2x^3 - 2x - 4 = 0

x 3 ( x + 2 ) 2 ( x + 2 ) = 0 ( x 3 2 ) ( x + 2 ) = 0 \implies x^3(x + 2) - 2(x + 2) = 0 \implies (x^3 - 2)(x + 2) = 0

x > 0 x 2 x = 2 1 3 a = 2 2 3 1 x > 0 \implies x \neq -2 \implies x = 2^{\frac{1}{3}} \implies a = \sqrt{2^{\frac{2}{3}} - 1} .

Let F C = l FC = l

m B E F = 90 θ m F E C = 90 + θ m\angle{BEF} = 90 - \theta \implies m\angle{FEC} = 90 + \theta and sin ( θ ) = a x \sin(\theta) = \dfrac{a}{x} \implies

Using the law of cosines on F E C \triangle{FEC} \implies l 2 = a 2 + 1 2 a cos ( 90 + θ ) = l^2 = a^2 + 1 - 2a\cos(90 + \theta) =

a 2 + 1 + 2 a sin ( θ ) = a^2 + 1 + 2a\sin(\theta) = a 2 + 1 + 2 a 2 x = 2 2 3 1 + 1 + ( 2 2 3 ) ( 2 2 3 1 ) = a^2 + 1 + \dfrac{2a^2}{x} = 2^{\frac{2}{3}} - 1+ 1 + (2^{\frac{2}{3}})(2^{\frac{2}{3}} - 1) =

2 2 3 ( 1 + 2 2 3 1 ) = 2 4 3 l = 2 2 3 2^ {\frac{2}{3}}(1 + 2^{\frac{2}{3}} - 1) = 2^{\frac{4}{3}} \implies l = 2^{\frac{2}{3}}

A F E C = 1 2 2 2 3 ( a sin ( 3 0 ) = \implies A_{\triangle{FEC}} = \dfrac{1}{2}2^{\frac{2}{3}}(a\sin(30^{\circ}) = 1 2 2 2 3 2 2 3 1 ( 1 2 ) = \dfrac{1}{2}2^{\frac{2}{3}}\sqrt{2^{\frac{2}{3}} - 1} (\dfrac{1}{2}) = 2 2 3 1 2 4 3 \dfrac{\sqrt{2^{\frac{2}{3}} - 1}}{2^{\frac{4}{3}}}

A F B E = 1 2 ( a ) ( 1 ) = 2 2 3 1 2 A_{\triangle{FBE}} = \dfrac{1}{2}(a)(1) = \dfrac{\sqrt{2^{\frac{2}{3}} - 1}}{2}

A F D B = 1 2 ( 1 ) ( 3 2 ) = 3 4 A_{\triangle{FDB}} = \dfrac{1}{2}(1)(\dfrac{\sqrt{3}}{2}) = \dfrac{\sqrt{3}}{4}

Let A F = m AF = m

Using the law of cosines on A D F \triangle{ADF} \implies

m 2 = 2 2 cos ( 12 0 ) = 2 + 2 ( 1 2 ) = 3 m = 3 m^2 = 2 - 2\cos(120^{\circ}) = 2 + 2(\dfrac{1}{2}) = 3 \implies m = \sqrt{3} \implies

A A D F = 1 2 3 ( sin ( 3 0 ) = 3 4 A_{\triangle{ADF}} = \dfrac{1}{2}\sqrt{3}(\sin(30^{\circ}) = \dfrac{\sqrt{3}}{4}

A = 2 2 3 1 2 4 3 + 2 2 3 1 2 + \implies A = \dfrac{\sqrt{2^{\frac{2}{3}} - 1}}{2^{\frac{4}{3}}} + \dfrac{\sqrt{2^{\frac{2}{3}} - 1}}{2} + 3 2 1.55339022 \dfrac{\sqrt{3}}{2} \approx \boxed{1.55339022}

Great problem, and a very neat writeup. I like how the quadrilateral splits into two distinct problems of highly differing difficulties - solving Δ A B F \Delta ABF is completely trivial, and solving Δ B C F \Delta BCF appears to involve doubling the cube!

Chris Lewis - 1 year, 4 months ago

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