Extend the above diagram to congruent circles where is a even positive integer.
In square , one of the vertices of square touches at and is tangent to circle at for each integer , where and the radius of each congruent circle is half the side of the square .
Let be the area of the shaded green region.
Find the value of for which
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Let n be a fixed even integer, a a side of square A B C D and x a side of square A J P 1 I .
Extending the diagram to even n congruent circles we obtain:
2 a = 2 x + ( n − 1 ) x + 2 x + 2 x ⟹ 4 a = ( 6 + 2 ( 2 n − 1 ) ) x ⟹
x = 6 + 2 ( 2 n − 1 ) 4 a = 2 ( 3 2 + ( 2 n − 1 ) ) 4 a = 3 2 + ( 2 n − 1 ) 2 2 a
C P 2 n + 1 = ( n − ( 2 n + 1 ) + 2 1 + 2 1 ) x = ( 2 2 2 n + 2 − 2 ) ( 2 n + 3 2 − 1 2 2 ) a =
2 n + 3 2 − 1 2 ( n + 2 − 1 ) a ⟹ A △ 1 = ( C P 2 n + 1 ) 2 = ( 2 n + 3 2 − 1 ) 2 2 ( n + 2 − 1 ) 2 a 2
A P 2 n = ( 2 + ( 2 n − 1 ) ) x = ( 2 n + 2 ( 2 − 1 ) ) ( 2 n + 3 2 − 1 2 2 ) a =
2 n + 3 2 − 1 2 ( n + 2 ( 2 − 1 ) ) a ⟹ A △ 2 = ( A P 2 n ) 2 = ( 2 n + 3 2 − 1 ) 2 2 ( n + 2 ( 2 − 1 ) ) 2 a 2
⟹ A △ 1 + A △ 2 = ( 2 n + 3 2 − 1 ) 2 2 ( 2 n 2 + 6 ( 2 − 1 ) n + 5 ( 3 − 2 2 ) ) a 2 ⟹ A T = a 2 − ( A △ 1 + A △ 2 ) ⟹
A △ A B C D A T = ( 2 n + 3 2 − 1 ) 2 8 n + 1 4 2 − 1 1 = 9 3 2 2 − 4 1
⟹ ( 1 2 8 2 − 1 6 4 ) n 2 + ( 8 6 0 − 6 2 0 2 ) n + 7 2 8 2 − 1 0 6 4 = 0 ⟹
n = 2 ( 1 2 8 2 − 1 6 4 ) − ( 8 6 0 − 6 2 0 2 ) ± ( 2 0 4 − 1 0 8 2 )
For ( + ) : n = 2 ( 1 2 8 2 − 1 6 4 ) − 6 5 6 + 5 1 2 2 = 2 ( 1 2 8 2 − 1 6 4 ) 4 ( 1 2 8 2 − 1 6 4 ) = 2
For ( − ) : n = 3 6 7 3 7 1 − 5 2 5 2 < 0 ∴ dropping negative irrational root ⟹ n = 2 .