Area Of A Quadrilateral!

Geometry Level 3

In the diagram above, A B D = 4 0 , B C D = 4 5 , B D C = 5 \angle{ABD} = 40^{\circ}, \angle{BCD} = 45^{\circ}, \angle{BDC} = 5^{\circ}

and A B = 6 AB = 6 .

Find the area of quadrilateral A B C D ABCD .

I found the above problem on YouTube, although my solution is different. Supposedly this problem was for 10 10 year old children in China.


The answer is 18.

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2 solutions

Nibedan Mukherjee
Apr 11, 2020

Very Nice Solution.

Rocco Dalto - 1 year, 2 months ago

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gracias.. !

nibedan mukherjee - 1 year, 2 months ago
Rocco Dalto
Apr 10, 2020

In right A B D \triangle{ABD} we have:

6 x = tan ( 5 0 ) x = 6 cot ( 5 0 ) y = 6 csc ( 5 0 ) \dfrac{6}{x} = \tan(50^{\circ}) \implies x = 6\cot(50^{\circ}) \implies y = 6\csc(50^{\circ})

Using the law of sines on B C D z sin ( 13 0 ) = 6 2 csc ( 5 0 ) \triangle{BCD} \implies \dfrac{z}{\sin(130^{\circ})} = 6\sqrt{2}\csc(50^{\circ}) \implies

z = 6 2 csc ( 5 0 ) sin ( 13 0 ) z = 6\sqrt{2}\csc(50^{\circ})\sin(130^{\circ}) (will simplify all later)

and

m sin ( 5 ) = 6 2 csc ( 5 0 ) m = 6 2 csc ( 5 0 ) sin ( 5 ) \dfrac{m}{\sin(5^{\circ})} = 6\sqrt{2}\csc(50^{\circ}) \implies m = 6\sqrt{2}\csc(50^{\circ})\sin(5^{\circ})

h = m 2 = 6 csc ( 5 0 ) sin ( 5 ) h = \dfrac{m}{\sqrt{2}} = 6\csc(50^{\circ})\sin(5^{\circ})

A A B D = 1 2 ( 6 ) ( 6 cot ( 5 0 ) ) = 18 cot ( 5 0 ) \implies A_{\triangle{ABD}} = \dfrac{1}{2}(6)(6\cot(50^{\circ})) = 18\cot(50^{\circ})

and

A B C D = 18 2 ( cot ( 5 0 ) sin ( 13 0 ) sin ( 5 ) A_{BCD} = 18\sqrt{2}(\cot(50^{\circ})\sin(130^{\circ})\sin(5^{\circ})

A A B C D = 18 ( cot ( 5 0 ) + 2 csc 2 ( 5 0 ) sin ( 13 0 ) sin ( 5 ) ) \implies A_{ABCD} = 18(\cot(50^{\circ}) + \sqrt{2}\csc^2(50^{\circ})\sin(130^{\circ})\sin(5^{\circ}))

and

cot ( 5 0 ) + 2 csc 2 ( 5 0 ) sin ( 13 0 ) sin ( 5 ) = \cot(50^{\circ}) + \sqrt{2}\csc^2(50^{\circ})\sin(130^{\circ})\sin(5^{\circ}) =

cot ( 5 0 ) + 2 csc 2 ( 5 0 ) sin ( 5 0 ) sin ( 5 ) = \cot(50^{\circ}) + \sqrt{2}\csc^2(50^{\circ})\sin(50^{\circ})\sin(5^{\circ}) =

1 sin ( 5 0 ) ( cos ( 5 0 ) + 2 sin ( 5 ) ) = \dfrac{1}{\sin(50^{\circ})}(\cos(50^{\circ}) + \sqrt{2}\sin(5^{\circ})) =

1 sin ( 5 0 ) ( cos ( 5 0 ) + 2 sin ( 4 5 4 0 ) ) = \dfrac{1}{\sin(50^{\circ})}(\cos(50^{\circ}) + \sqrt{2}\sin(45^{\circ} - 40^{\circ})) =

1 sin ( 5 0 ) ( cos ( 5 0 ) + cos ( 4 0 ) sin ( 4 0 ) ) = \dfrac{1}{\sin(50^{\circ})}(\cos(50^{\circ}) + \cos(40^{\circ}) - \sin(40^{\circ})) =

1 sin ( 5 0 ) ( cos ( 5 0 ) + sin ( 5 0 ) cos ( 5 0 ) ) = 1 \dfrac{1}{\sin(50^{\circ})}(\cos(50^{\circ}) + \sin(50^{\circ}) - \cos(50^{\circ})) = 1

A A B C D = 18 \implies A_{ABCD} = \boxed{18} .

nice approach!

nibedan mukherjee - 1 year, 2 months ago

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