In the picture, the arc AC is one fourth of a circumference of center D and the arc AB is one eighth of a circumference of center C. The segment AD has a length of 2 cm. What is the area in cm² of the green region?
Problem from OBMEP 2017.
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Unshaded area:
= 8 1 ( 2 2 ) 2 π − ( 4 1 ⋅ 2 2 π − 2 1 ⋅ 2 2 ) = 8 1 ⋅ 8 π − ( 4 1 ⋅ 4 π − 2 ) = π − ( π − 2 ) = π − π + 2 = 2
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Since arc A C is one fourth of circumference of center D , then ∠ C D A = 9 0 ∘ . Since arc A B is 8 1 of circumference of center C , ∠ A C B = 4 5 ∘ .
By Pythagorean theorem, A C = 2 2 + 2 2 = 8 = 2 2 .
It follows that B C = A C = 2 2 .
Consider sector C D A : The area of sector C D A is A = 4 1 π ( 2 2 ) = π
Consider sector A C B : The area of sector A C B is A = 3 6 0 4 5 π ( 2 2 ) 2 = 8 1 π ( 4 ) ( 2 ) = π
Area of the yellow segment is π − 2 1 ( 2 ) ( 2 ) = π − 2
Finally, the area of the green region is π − ( π − 2 ) = π − π + 2 = 2