Question

For every real number $x,$ let $\lfloor x\rfloor$ denote the greatest integer not exceeding $x,$ and let $f(x)=\lfloor x\rfloor\left(2014^{\,x-\lfloor x\rfloor}-1\right).$ The set of all numbers $x$ such that $1\le x\lt2014$ and $f(x)\le1$ is a union of disjoint intervals. What is the sum of the lengths of those intervals?

$1$

$\dfrac{\log2015}{\log2014}$

$\dfrac{\log2014}{\log2013}$

$\dfrac{2014}{2013}$

$2014^{1/2014}$

Answer: A

Concepts:floor and ceiling functions logarithm telescoping

Difficulty rating: 2170

Solution:

Write $x=n+r$ with integer $n$ ( $1\le n\le2013$ ) and $0\le r\lt1.$ Then $f(x)=n\left(2014^{\,r}-1\right),$ and $f(x)\le1$ becomes $2014^{\,r}\le1+\dfrac1n,$ i.e. $0\le r\le\log_{2014}\dfrac{n+1}{n}.$

Each $n$ contributes an interval of length $\log_{2014}\dfrac{n+1}{n},$ so the total is $\begin{gathered} \sum_{n=1}^{2013}\log_{2014}\dfrac{n+1}{n}\\ =\log_{2014}\!\left(\dfrac21\cdot\dfrac32\cdots\dfrac{2014}{2013}\right)\\ =\log_{2014}2014=1. \end{gathered}$

Thus, the correct answer is A.

2014 AMC 12A Problem 21

Answer: A

Difficulty rating: 2170

Solution:

Problem 21 in Other Years