A circle and equilateral triangle are inscribed in a semicircle with radius 1 and the circle is tangent to the semicircle and equilateral triangle as shown above and one of the vertices of the equilateral triangle is at the center of the semicircle.
If the area of the circle A c = α ( β − λ α ) π , where α , β and λ are coprime positive integers, find α + β + λ .
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Are there any typos in the second line? Should A B E be A B C and should N R E be N O E ?
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Yes there is. It should be:
m ∠ C F D = 6 0 ∘ ⟹ m ∠ A F C = 1 2 0 ∘ and letting m ∠ N O E = θ ⟹
1 2 0 ∘ = 2 1 ( m ( N B E ) − m ( N R E ) ) = 2 1 ( 3 6 0 ∘ − 2 θ ) ⟹
2 4 0 ∘ = 3 6 0 ∘ − 2 θ ⟹ θ = 6 0 ∘ ⟹ m ∠ N O F = 3 0 ∘ ⟹
∠ O F N = 6 0 ∘
and I forgot to put the R in the diagram for N R E . I'll add it. Thanks!
Let the equilateral triangle be A B C , where C is the center of the semicircle, the tangent points to equilateral triangle, the semicircle diameter, and the semicircle arc be M , N , and Q respectively, and the radius of the circle be r . Then O M = O N = O Q = r , where O is the center of the circle, and they are perpendicular to the respective line.
We note that C , O , and Q are colinear. Since △ A B C is equilateral, ∠ A C B = 6 0 ∘ , ⟹ ∠ A C N = 1 8 0 ∘ − 6 0 ∘ = 1 2 0 ∘ . Due to symmetry, ∠ O C M = ∠ O C N = 6 0 ∘ . Then
Q O + O C Q O + O N csc ∠ O C N r + r csc 6 0 ∘ r ⟹ A c = Q C = Q C = 1 = 1 + csc 6 0 ∘ 1 = 1 + 3 2 1 = 3 + 2 3 = 4 − 3 3 ( 2 − 3 ) = 2 3 − 3 = π r 2 = ( 2 3 − 3 ) 2 π = ( 2 1 − 1 2 3 ) π = 3 ( 7 − 4 3 ) π
Therefore α + β + γ = 3 + 7 + 4 = 1 4 .
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m ∠ C F D = 6 0 ∘ ⟹ m ∠ A F C = 1 2 0 ∘ and letting m ∠ N O E = θ ⟹
1 2 0 ∘ = 2 1 ( m ( N B E ) − m ( N R E ) ) = 2 1 ( 3 6 0 ∘ − 2 θ ) ⟹
2 4 0 ∘ = 3 6 0 ∘ − 2 θ ⟹ θ = 6 0 ∘ ⟹ m ∠ N O F = 3 0 ∘ ⟹
∠ O F N = 6 0 ∘ ⟹ y r = tan ( 6 0 ∘ ) = 3 , where y = N F
⟹ y = 3 r ⟹ m = 3 2 r ⟹ B F = r + m = 3 2 + 3 r = 1 ⟹
r = 2 + 3 3 = 2 3 − 3 ⟹ A c = π ( 2 3 − 3 ) 2 = 3 ( 7 − 4 3 ) π
α ( β − λ α ) π ⟹ α + β + λ = 1 4