In the diagram above square A B C D has side length 1 and the pink, red and blue circle are tangent to each other and tangent to the green circle and B C is tangent to the pink circle and blue circle and A B is tangent to the pink and red circle.
Let S be the sum of the areas of the pink, red and blue circles.
If S = β α π ( λ − ω β ) , where α , β , λ and ω are coprime positive integers, find α + β + λ + ω .
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Let the radius of the pink circle be R and the radius of the red and blue circles be r . Add E and F as follows:
As a diagonal of a unit square, B D = 2 . As a hypotenuse of right △ B F O with O F = R , B O = 2 R . Therefore, B D = B O + O E + E D = 2 R + R + 1 = 2 , which solves to R = 3 − 2 2 .
By Descartes' Theorem (special case with a straight line), r 1 = 1 + R 1 + 2 1 ⋅ R 1 = 1 + 3 − 2 2 1 + 2 1 ⋅ 3 − 2 2 1 = 3 − 2 2 2 , so that r = 2 1 ( 3 − 2 2 ) .
Therefore, S = π R 2 + 2 π r 2 = π ( ( 3 − 2 2 ) 2 + 2 ( 2 1 ( 3 − 2 2 ) ) 2 ) = 2 3 ( 1 7 − 1 2 2 ) , so α = 3 , β = 2 , λ = 1 7 , ω = 1 2 , and α + β + λ + ω = 3 4 .
r p i n k = 3 − 2 2 r r e d = r b l u e = 2 3 − 2 2 = 2 r p i n k S = π ( r p i n k 2 + r r e d 2 + r b l u e 2 ) = 2 3 π r p i n k 2 = 2 3 π ( 3 − 2 2 ) 2 = 2 3 π ( 1 7 − 1 2 2 )
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Note: We just need to find R then r since r = r ′ using the symmetry about B D .
B D = 2 = 1 + R + 2 R = 1 + ( 2 + 1 ) R ⟹ R = 2 + 1 2 − 1 = 3 − 2 2
Using △ O ′ E O ⟹ m 2 = ( R + r ) 2 − ( R − r ) 2 = 4 R r ⟹ m = 2 R r
and
Using △ F O ′ D ⟹ ( 1 + r ) 2 = ( 1 − r ) 2 + ( 1 − R − m ) 2 ⟹
4 r = ( 1 − R ) 2 − 2 m ( 1 − r ) + m 2 = ( 1 − R ) 2 − 4 ( 1 − R ) R r + 4 R r ⟹
4 ( 1 − R ) r = ( 1 − R ) 2 − 4 R r ( 1 − R ) ⟹ 4 r = 1 − R − 4 R r ⟹
4 r + 4 R r + R = 1 ⟹ ( R + 2 r ) 2 = 1 ⟹ R + 2 r = 1 ⟹
r = 2 1 − R = 2 1 − 3 − 2 2 = 2 1 − ( 2 − 1 ) 2 = 2 2 − 2 = 2 2 ( 2 − 1 )
⟹ r = 4 2 ( 3 − 2 2 ) = 2 3 − 2 2 = 2 R ⟹ S = π R 2 + 2 π ( 2 R ) 2 =
2 3 π ( 3 − 2 2 ) 2 = 2 3 π ( 1 7 − 1 2 2 ) = β α π ( λ − ω β ) ⟹ α + β + λ + ω = 3 4 .