It's all about number theory! (3)

A list of arbitrary non-negative different integers contains numbers 0 and 2015. What's the minimum possible value of the average of this list of numbers?


See also It's all about number theory! (2)
This problem is not original. It is taken from this year's first round of the Regional Mathematics Olympiad in Colombia.


The answer is 62.

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1 solution

the average x \overline{x} where minimal while sum of the elements of the list where minimal, also i can't have the same number 2 times in the list, then the x \overline{x} can have here minimun value whene x = 2015 + k = 0 n k n + 1 \overline{x}=\frac{2015+\sum_{k=0}^{n}{k}}{n+1} m i n { x } = m i n { 2015 + k = 0 n k n + 1 } min\{\overline{x}\}=min\{\frac{2015+\sum_{k=0}^{n}{k}}{n+1}\} m i n { x } = m i n { 2015 n + 1 + 1 2 n } min\{\overline{x}\}=min\{\frac{2015}{n+1}+\frac{1}{2}n\} m i n { x } d d n ( 2015 n + 1 + 1 2 n ) = 0 min\{\overline{x}\}\rightarrow\frac{d}{dn}(\frac{2015}{n+1}+\frac{1}{2}n)=0 m i n { x } 1 2 = 2015 ( n + 1 ) 2 min\{\overline{x}\}\rightarrow\frac{1}{2}=\frac{2015}{(n+1)^2} m i n { x } n = 2015 2 1 min\{\overline{x}\}\rightarrow n=\left\lfloor{\sqrt{2015*2}}-1\right\rfloor m i n { x } n = 62 min\{\overline{x}\}\rightarrow n=62

Three observations:

1) In your reasoning, the list has n + 2 n+2 terms, not n + 1 n+1 . Small mistake.

2) Why do you use the floor function instead of the ceiling function?

3) Can you differentiate a non-real valued function?

Is there a non-calculus way to solve this problem?

Miguel Vásquez Vega - 5 years, 8 months ago

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