In the diagram above, ∠ A B D = 4 0 ∘ , ∠ B C D = 4 5 ∘ , ∠ B D C = 5 ∘
and A B = 6 .
Find the area of quadrilateral A B C D .
I found the above problem on YouTube, although my solution is different. Supposedly this problem was for 1 0 year old children in China.
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Very Nice Solution.
In right △ A B D we have:
x 6 = tan ( 5 0 ∘ ) ⟹ x = 6 cot ( 5 0 ∘ ) ⟹ y = 6 csc ( 5 0 ∘ )
Using the law of sines on △ B C D ⟹ sin ( 1 3 0 ∘ ) z = 6 2 csc ( 5 0 ∘ ) ⟹
z = 6 2 csc ( 5 0 ∘ ) sin ( 1 3 0 ∘ ) (will simplify all later)
and
sin ( 5 ∘ ) m = 6 2 csc ( 5 0 ∘ ) ⟹ m = 6 2 csc ( 5 0 ∘ ) sin ( 5 ∘ )
h = 2 m = 6 csc ( 5 0 ∘ ) sin ( 5 ∘ )
⟹ A △ A B D = 2 1 ( 6 ) ( 6 cot ( 5 0 ∘ ) ) = 1 8 cot ( 5 0 ∘ )
and
A B C D = 1 8 2 ( cot ( 5 0 ∘ ) sin ( 1 3 0 ∘ ) sin ( 5 ∘ )
⟹ A A B C D = 1 8 ( cot ( 5 0 ∘ ) + 2 csc 2 ( 5 0 ∘ ) sin ( 1 3 0 ∘ ) sin ( 5 ∘ ) )
and
cot ( 5 0 ∘ ) + 2 csc 2 ( 5 0 ∘ ) sin ( 1 3 0 ∘ ) sin ( 5 ∘ ) =
cot ( 5 0 ∘ ) + 2 csc 2 ( 5 0 ∘ ) sin ( 5 0 ∘ ) sin ( 5 ∘ ) =
sin ( 5 0 ∘ ) 1 ( cos ( 5 0 ∘ ) + 2 sin ( 5 ∘ ) ) =
sin ( 5 0 ∘ ) 1 ( cos ( 5 0 ∘ ) + 2 sin ( 4 5 ∘ − 4 0 ∘ ) ) =
sin ( 5 0 ∘ ) 1 ( cos ( 5 0 ∘ ) + cos ( 4 0 ∘ ) − sin ( 4 0 ∘ ) ) =
sin ( 5 0 ∘ ) 1 ( cos ( 5 0 ∘ ) + sin ( 5 0 ∘ ) − cos ( 5 0 ∘ ) ) = 1
⟹ A A B C D = 1 8 .
nice approach!
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