Extend the diagram above to an infinite number of circles. For each integer n ≥ 1 , circle w n is tangent to w n − 1 and tangent to the line y = x and the positive x -axis.
Let C n and A n be the circumference and the area of the n th circle respectively. Find:
∑ n = 1 ∞ A n ( ∑ n = 1 ∞ C n ) 2
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For any measure of ∠ O , as the pattern repeats, the ratio of the shortest distance of any circle (for example: O A = l 1 ) to the apex O and the diameter of the circle ( A B = d 1 ) is a constant. Let the constant be k then:
d 1 l 1 = d 2 l 2 = d 3 l 3 = ⋯ = k
We note that:
l n − 1 k d n − 1 d n ⟹ r n = d n + l n = d n + k d n = 1 + k k d n − 1 = 1 + k k r n − 1 = ( 1 + k k ) n − 1 r 1 where r n = 2 d n , the radius of n th circle.
Then we have:
n = 1 ∑ ∞ C n n = 1 ∑ ∞ A n ⟹ ∑ n = 1 ∞ A n ( ∑ n = 1 ∞ C n ) 2 = n = 1 ∑ ∞ 2 π r n = n = 0 ∑ ∞ 2 π ( 1 + k k ) n r 1 = 1 − 1 + k k 2 π r 1 = 2 π r 1 ( 1 + k ) = n = 1 ∑ ∞ π r n 2 = 1 − ( 1 + k k ) 2 π r 1 2 = 1 + 2 k π r 1 2 ( 1 + k ) 2 = 1 + 2 k π r 1 2 ( 1 + k ) 2 ( 2 π r 1 ( 1 + k ) ) 2 = 4 π ( 1 + 2 k )
Let us find k for ∠ O = 4 π . Then k = d 1 l 1 = 2 r 1 l 1 = 2 r 1 r 1 csc 8 π − r 1 = 2 csc 8 π − 1 = 2 4 + 2 2 − 1 . Then
∑ n = 1 ∞ A n ( ∑ n = 1 ∞ C n ) 2 = 4 π ( 1 + 2 k ) = 2 π 4 + 2 2 ≈ 3 2 . 8
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O w 0 = 4 + 2 2 R 1
△ O A 1 w 0 ∼ △ w 1 A 2 w 0 ⟹ R 1 4 + 2 2 R 1 = R 1 − R 2 R 1 + R 2 ⟹
( 4 + 2 2 − 1 ) R 1 = ( 4 + 2 2 + 1 ) R 2 ⟹ R 2 = 4 + 2 2 + 1 4 + 2 2 − 1 R 1
R 3 = 4 + 2 2 + 1 4 + 2 2 − 1 R 2 = ( 4 + 2 2 + 1 4 + 2 2 − 1 ) 2 R 1
In General: R n = ( 4 + 2 2 + 1 4 + 2 2 − 1 ) n − 1 R 1
⟹ A n = π R n 2 = π R 1 2 ( ( 4 + 2 2 + 1 4 + 2 2 − 1 ) 2 ) n − 1
⟹ A = ∑ n = 1 ∞ A n = π R 1 2 ∑ n = 1 ∞ ( ( 4 + 2 2 + 1 4 + 2 2 − 1 ) 2 ) n − 1
= 4 4 + 2 2 ( 4 + 2 2 + 1 ) 2 π R 1 2
and
C n = 2 π R n = 2 π R 1 ( 4 + 2 2 + 1 4 + 2 2 − 1 ) n − 1
⟹ C = ∑ n = 1 ∞ C n = 2 π R 1 ∑ n = 1 ∞ ( 4 + 2 2 + 1 4 + 2 2 − 1 ) n − 1 =
( 4 + 2 2 ) π R 1 ⟹ C 2 = ( 4 + 2 2 ) 2 π 2 R 1 2
⟹ A C 2 = 4 π 4 + 2 2 ≈ 3 2 . 8 3 7 5 0 8 8 9 5 2 6 4 9 8 8 4 .