To Log or Not to Log

Algebra Level 4

Given that log 10 2 0.3010 \log_{10} 2\approx0.3010 , find the smallest positive integer n > 50 n>50 , such that the number of digits of the two numbers 5 n 5^n and 5 n + 1 5^{n+1} are identical.


The answer is 53.

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1 solution

A P
May 29, 2016

We can find the number of digits in a number by taking the floor of a log of a number, and adding one, as such: l o g x + 1 = \lfloor logx\rfloor+1= number of digits in x x . Therefore, we have the number of digits in 5 x = x l o g 5 + 1 5^x=\lfloor xlog5 \rfloor+1 , because of the property l o g x y z = z l o g x y log_xy^z=zlog_xy . l o g 5 = l o g 10 l o g 2 log5=log10-log2 , because of the property l o g x l o g y = l o g x y logx-logy=log\frac{x}{y} , and from there it is just brute force by calculating out each scenario for x > 50 x>50 .

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