In the diagram above the two regions are labeled R and B .
If the area of the pink region A P = 2 1 and A R = β β α ( α arctan ( β ) − β ω − β arctan ( β ) ) , where α , β ω are coprime positive integers, find α + β + ω .
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By the Pythagorean Theorem on △ B D E , B E = B D 2 + D E 2 = ( 2 a ) 2 + a 2 = 5 a , so tan ∠ B E D = a 2 a = 2 and sin ∠ B E D = 5 a 2 a = 5 2 .
The area of the sector formed by E B D is A sec E B D = 2 π m ∠ B E D π a 2 = 2 1 a 2 arctan ( 2 ) .
The area of △ E B D is A △ E B D = 2 1 a 2 sin ∠ B E D = 5 1 a 2 .
Then A p = A sec E B D − A △ E B D = 2 1 a 2 arctan ( 2 ) − 5 1 a 2 = 2 1 , which solves to a 2 = 5 arctan ( 2 ) − 2 5 .
And A R = A F C D E − A △ F B E − A sec E B D = a 2 − 4 1 a 2 − 2 1 a 2 arctan ( 2 ) = 4 3 − 2 arctan ( 2 ) ⋅ 5 arctan ( 2 ) − 2 5 = 2 2 5 ( 5 arctan ( 2 ) − 2 3 − 2 arctan ( 2 ) ) .
Therefore, α = 5 , β = 2 , ω = 3 , and α + β + ω = 1 0 .
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△ A B E ∼ △ F M E ⟹ 1 2 = F M a ⟹ F M = 2 a ⟹ A △ F M E = 4 a 2
and tan ( θ ) = 2 1 ⟹ θ = arctan ( 2 1 ) ⟹ A s e c t o r ( F P E ) = 2 1 arctan ( 2 1 ) a 2 ⟹
A B = A △ F M E − A s e c t o r ( F P E ) = ( 4 1 − 2 1 arctan ( 2 1 ) ) a 2 ⟹
A R = a 2 − 4 π a 2 − A B = ( 4 3 + 2 1 arctan ( 2 1 ) − 4 π ) a 2
To Find a 2 using A P = 2 1 :
B E = 5 and tan ( λ ) = tan ( 2 π − θ ) = tan ( θ ) 1 = 2 ⟹
A s e c t o r ( P D E ) = 2 1 arctan ( 2 ) a 2 and h = a sin ( λ ) = a cos ( θ ) = 5 2 a ⟹
A △ P D E = 2 1 ( a ) ( 5 2 a ) = 5 a 2 ⟹ A P = 2 1 = A s e c t o r ( P D E ) − A △ P D E =
( 2 5 5 arctan ( 2 ) − 2 ) a 2 ⟹ a 2 = 5 arctan ( 2 ) − 2 5
Using the fact that θ = arctan ( 2 1 ) and λ = 2 π − θ = arctan ( 2 ) ⟹
θ = 2 π − arctan ( 2 ) ⟹ A R = 4 5 ( 5 arctan ( 2 ) − 2 3 − 2 arctan ( 2 ) ) =
β β α ( α arctan ( β ) − β ω − β arctan ( β ) ) ⟹ α + β + ω = 1 0 .