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