Bye 2018

Algebra Level 4

Let A A be the digits of 201 8 2018 2018^{2018} in decimal representation. Find the integer k k such that the following equation 2 k + 1 < A 101 < 2 k + 3 2k+1< \frac{A}{101} < 2k+3 is true


The answer is 32.

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1 solution

Jordan Cahn
Dec 21, 2018

The number of digits in a number N N (that is not a power of 10) is simply log 10 N \left\lceil \log_{10} N\right\rceil . A 101 = 1 101 log 10 201 8 2018 = 2018 101 log 10 ( 2.018 × 1 0 3 ) = 2018 101 ( 3 + log 10 2.018 ) 20 ( 3 + log 10 2 ) 60 + log 10 2 20 60 + 6 = 66 \begin{aligned} \frac{A}{101} &= \left\lceil\frac{1}{101}\log_{10}2018^{2018}\right\rceil \\ &= \left\lceil\frac{2018}{101}\log_{10}{(2.018\times 10^3})\right\rceil \\ &= \left\lceil \frac{2018}{101}\left(3 + \log_{10}2.018 \right)\right\rceil \\ &\approx \left\lceil 20(3+\log_{10}2)\right\rceil \\ &\approx 60 + \log_{10}2^{20}\\ &\approx 60 + 6 = 66 \end{aligned} Thus, we need k k such that 2 k + 1 < 66 < 2 k + 3 2k+1<66<2k+3 and k = 32 k=\boxed{32}

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