Recursive Sequence

Algebra Level pending

A sequence of real numbers f n f_n is defi ned recursively as follows:

f n = f n 1 + 1 f n 2 , n = 0 , 1 , 2 , . \large f_n =\frac{f_{n-1} +1}{f_{n-2}} ,\ \ n =0,1,2,\ldots.

Given that f 0 = 24.8 f_0 = 24.8 and f 1 = 36.9 f_1 = 36.9 , find 10 × f 4445 10 \times f_{4445} .


The answer is 248.

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1 solution

Ossama Ismail
Mar 13, 2017

Direct substitution in the recurrence relation f n = f n 1 + 1 f n 2 , n = 0 , 1 , 2 , \large f_n =\frac{f_{n-1} +1}{f_{n-2}} ,\ \ n =0,1,2,\cdots gives the following :-

f 0 = 24.8 f 1 = 36.9 f 2 = 1.52823 f 3 = 0.0685156 f 4 = 0.699187 f 5 = 24.8 f 6 = 36.9 f 7 = 1.52823 f 8 = 0.0685156 f 9 = 0.699187 f 10 = 24.8 f 11 = 36.9 f 12 = 1.52823 f 13 = 0.0685156 f 14 = 0.699187 f 15 = 24.8 = \begin{aligned} \color{#D61F06} f_0&=24.8 \\ f_1&=36.9 \\ f_2&=1.52823 \\ f_3&=0.0685156\\ f_4&=0.699187\\ \color{#D61F06}f_5&=24.8\\ f_6&=36.9\\ f_7&=1.52823\\ f_8&=0.0685156\\ f_9&=0.699187\\ \color{#D61F06}f_{10}&=24.8\\ f_{11}&=36.9\\ f_{12}&=1.52823\\ f_{13}&=0.0685156\\ f_{14}&=0.699187\\ \color{#D61F06}f_{15}&=24.8\\ \cdots&=\cdots\\ \end{aligned}

The above sequence has cycle with length = 5 = 5

f 4445 = f 4445 % 5 = f 0 = 24.8 f_{4445} = f_{4445 \% 5} = f_0 = 24.8

Answer = 10 × 24.8 = 248 = 10 \times 24.8 = 248

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