In right △ A B C , m ∠ A B C = 3 0 ∘ , A D = x , D B = 3 x and the quarter of the circle has radius C E = 2 x − 2 .
If the sum of the areas A 1 + A 2 = a ( b + c a − ( d + e a ) π ) , where a , b , c , d and e are coprime positive integer, find a + b + c + d + e .
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Interestingly, H E = A D .
I found the same answer, however, I was a little bit confused by the sentence "coprime positive integers" because a and b, or b and e are not coprime integers each other.
A F = 2 x − ( 2 x − 2 ) = 2 , G D = 2 3 x , A G = 2 x ⟹
( C D ) 2 = 4 ( x − 1 ) 2 = 1 6 x 2 − 3 2 x + 1 6 = ( 2 x − 2 x ) 2 + 4 3 x 2 = 1 2 x 2 ⟹ x 2 − 8 x + 4 = 0 ⟹ x = 4 ± 2 3
x = 4 − 2 3 ⟹ 2 ( x − 1 ) = 2 ( 3 − 2 3 ) < 0 ∴ choose x = 4 + 2 3
⟹ radius r = 2 x − 2 = 2 ( 3 + 2 3 )
For A 2 :
Let θ = m ∠ D C E
⟹ tan ( θ ) = 3 ⟹ θ = 3 π ⟹ A s e c t o r C D E = 2 1 θ r 2 = 2 1 ( 3 π ) ( 4 ) ( 3 + 2 3 ) 2 =
2 π ( 7 + 4 3 )
and A △ C D I = 2 1 ( 2 3 x ) ( 2 3 x ) = 8 3 3 x 2 = 8 3 3 ( 4 + 2 3 ) 2 = 2 3 3 ( 7 + 4 3 )
Let R 2 be region D E I ⟹ A R 2 = A s e c t o r C D E − A △ C D I = 2 ( 7 + 4 3 ) ( 4 π − 3 3 )
B C = 2 3 x ⟹ B E = 2 3 x − ( 2 x − 2 ) = 2 ( ( 3 − 1 ) x + 1 ) =
2 ( 2 3 + 3 ) and △ A D G ∼ △ H B E ⟹ 3 1 = 2 ( 2 3 + 3 ) H E ⟹ H E = 3 4 3 + 6 = 4 + 2 3 = x and D I = 2 3 x = 3 ( 2 + 3 ) and I E = C E − C I =
( 2 4 − 3 ) x − 2 = 3 + 2 3 ⟹ area of trapezoid
A D H E I = 2 1 ( 5 ) ( 2 + 3 ) ( 3 + 2 3 ) = 2 5 ( 1 2 + 7 3 )
⟹ A 2 = A D H E I − A R 2 = 4 8 + 2 8 3 − ( 1 4 + 8 3 ) π
For A 1 :
m ∠ F C D = 6 π ⟹ A s e c t o r C D F = 2 1 ( 6 π ) ( 4 ) ( 3 + 2 3 ) 2 = π ( 7 + 4 3 )
and A △ D G C = 2 3 3 ( 7 + 4 3 )
Let R 1 be region F D G ⟹ A R 1 = A s e c t o r C D F − A △ D G C = 2 ( 7 + 4 3 ) ( 2 π − 3 3 )
and A △ A D G = 8 3 x 2 = 2 3 ( 7 + 4 3 )
⟹ A 1 = A △ A D G − A R 1 = ( 7 + 4 3 ) ( 2 3 − π ) = 2 4 + 1 4 3 − ( 7 + 4 3 ) π
⟹ A 1 + A 2 = 7 2 + 4 2 3 − ( 2 1 + 1 2 3 ) π = 3 ( 2 4 + 1 4 3 − ( 7 + 4 3 ) π ) =
a ( b + c a − ( d + e a ) π ) ⟹ a + b + c + d + e = 5 2 .
I found the same answer, however, I was a little bit confused by the sentence "coprime positive integers" because a and b, or b and e are not coprime integers each other.
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Coprime means the greatest common factor of all the integers is 1. Which is the case in this problem. Your thinking of pairwise positive integers.
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Let the point to the left of E be H , and let B E = y .
Since ∠ A B C = 3 0 ° and A B = 4 x , from △ A B C we have A C = 2 x and B C = 2 3 x = 2 x − 2 + y .
By the tangent-secant theorem , ( 3 x ) 2 = y ( 4 x − 4 + y ) .
These two equations solve to x = 4 + 2 3 and y = 6 + 4 3 .
That means A C = 2 x = 2 ( 4 + 2 3 ) = 8 + 4 3 and E C = 2 x − 2 = 2 ( 4 + 2 3 ) − 2 = 6 + 4 3 .
Since ∠ H B E = 3 0 ° and B E = y = 6 + 4 3 , from △ H B E we have H E = 3 3 ( 6 + 4 3 ) = 4 + 2 3 .
The sum of the areas A 1 + A 2 is the difference between the areas of trapezoid A H E C and the quarter circle, so:
A 1 + A 2 = 2 1 ( A C + H E ) ⋅ E C − 4 1 π ⋅ E C 2
A 1 + A 2 = 2 1 ( 8 + 4 3 + 4 + 2 3 ) ⋅ ( 6 + 4 3 ) − 4 1 π ⋅ ( 6 + 4 3 ) 2
A 1 + A 2 = 3 ( 2 4 + 1 4 3 − ( 7 + 4 3 ) π ) .
Therefore, a = 3 , b = 2 4 , c = 1 4 , d = 7 , e = 4 , and a + b + c + d + e = 5 2 .