There is a race being held in the city of Mathville, the capital city of Mathematica. The race commences at the city centre. The competitors must first run 1 hectometre east, then 1 hectometre north, then hectometres west, then hectometres south, hectometres east and so forth where the th side of this spiral is hectometers, where denotes the th Fibonacci number . Assuming the race goes on infinitely, the distance displaced by the competitors can be represented as metres . Find .
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Let the starting point of the race the city center be the origin O ( 0 , 0 ) of the complex plane. Let east and west direction be the positive and negative real axis respectively, and north and south, the positive and negative imaginary axis respectively. Let each straight run or side length with direction be denoted by complex number a n , where n is a positive integer. Then a 1 = 1 , a 2 = 2 1 i , a 3 = − 3 1 , a 4 = − 5 1 i , ... a n = F n i n − 1 .
Let s n = k = 1 ∑ n a k . We note that |s_n| is the distance between origin and the end of n th side. Then, we have:
n → ∞ lim s n = n → ∞ lim k = 1 ∑ n F k i k − 1 = k = 0 ∑ ∞ F 2 k + 1 ( − 1 ) k + i k = 1 ∑ ∞ F 2 k ( − 1 ) k + 1 = 0 . 6 4 4 3 5 8 3 3 8 + 0 . 7 5 7 2 0 4 3 7 5 i By numerical method (in hectometers)
Now l = 1 0 0 ∣ s ∞ ∣ = 1 0 0 × 0 . 9 9 4 2 6 1 6 0 2 m, ⟹ ⌊ l ⌋ = 9 9 m.