b l u e area with the g r e e n area.
In the figure, four circles are inscribed in a square. The circles are tangent to each other as well as to the sides of the square and to the black line segment. The two red circles are identical. Compare the
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Can you help me understand the geometry behind:
r 3 + ( r 1 + r 3 ) 2 − ( r 1 − r 3 ) 2 + r 1 = 1
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Thanks. I hated to draw the figure.
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@Chew-Seong Cheong – Thank you for posting your solution. Happy new year!
Thank you both.
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Let the side length of the square be 1 , the radii of the white, yellow and red circles be r 1 , r 2 , and r 3 respectively.
We note the side length of the square is given by:
r 3 + ( r 1 + r 3 ) 2 − ( r 1 − r 3 ) 2 + r 1 r 3 + 2 r 3 r 1 + r 1 ( r 3 + r 1 ) 2 ⟹ r 3 + r 1 = 1 = 1 = 1 = 1 . . . ( 1 )
The diagonal of the square is given by:
( 2 + 1 ) ( r 2 + r 1 ) ⟹ r 2 = 2 = 2 − 2 − r 1 . . . ( 2 )
The length of the black line segment is given by both ℓ = 2 r 3 cot 2 1 3 5 ∘ + 4 r 3 r 1 and ℓ = 2 r 2 cot 2 4 5 ∘ . Therefore, we have:
r 3 cot 2 1 3 5 ∘ + 2 r 3 r 1 ( 2 − 1 ) r 3 + 1 − r 3 − r 1 ( 2 − 2 ) r 3 + 1 − r 1 + ( 2 + 1 ) r 1 ( 2 − 2 ) r 3 + 1 + 2 r 1 ( 2 − 2 ) ( 1 − r 1 ) 2 + 1 + 2 r 1 ( 2 − 2 ) ( 1 − 2 r 1 + r 1 ) + 1 + 2 r 1 2 ( 2 − 1 ) r 1 + 2 ( 2 − 2 ) r 1 − 1 ⟹ 2 r 1 + 2 2 r 1 − 2 − 1 ⟹ r 1 = 2 3 + 2 1 − 2 + 2 r 2 = 2 1 − 2 3 + 2 + 2 r 3 = 2 5 + 2 1 − 2 + 2 − 2 ( 3 + 2 − 2 2 + 2 ) = r 2 cot 2 4 5 ∘ = ( 2 + 1 ) r 2 = ( 2 + 1 ) ( r 2 + r 1 ) = 2 = 2 = 2 = 0 = 0 ≈ 0 . 3 5 9 3 4 7 7 1 6 1 6 4 ≈ 0 . 2 2 6 4 3 8 7 2 1 4 6 3 ≈ 0 . 1 6 0 4 3 5 3 4 8 7 8 4 Note that r 3 + 2 r 3 r 1 + r 1 = 1 and ( 2 + 1 ) ( r 2 + r 1 ) = 2 and r 3 + r 1 = 1 From (2) From (1)
Note that the area of the blue triangle is Δ b l u e = ( 2 + 1 ) 2 r 2 2 and then the blue area A b l u e = Δ b l u e − π r 2 2 ≈ 0 . 1 3 7 7 6 6 0 7 9 5 2 3 and the green area A g r e e n = 1 − Δ b l u e − π ( r 1 2 + 2 r 3 2 ) ≈ 0 . 1 3 3 7 4 7 9 7 6 3 2 7 . Therefore
A b l u e > A g r e e n