2023 IOQM/Problem 11

Revision as of 11:20, 2 May 2024 by L13832 (talk | contribs) (Solution)

Problem

A positive integer $m$ haas the property that $m^2$ is expressible in the form $4n^2-5n+16$, where n is an integer. Find the maximum value of $|m-n|$.

Solution

$m^2=4n^2-5n+16$. Now we try to complete the square, multiplying by $4$ and $9$ won't complete the square but on multiplication with $16$ we get $(8n-5)^2+231=(4m)^2$ $\Rightarrow (8n-5-4m)(8n-5+4m)=-231$ or $(4m-8n+5)(4m+8n-5)=231$.

  • $\textbf{Case I}$: $(4m-8n+5)=1$ and $(4m+8n-5)=231$ $\Rightarrow$ $m=29$ and $n=15$, $|m-n|=14$.
  • $\textbf{Case II}$: $(4m-8n+5)=3$ and $(4m+8n-5)=77$ $\Rightarrow$ $m=10$ and $n=-4$,$|m-n|=14$.
  • $\textbf{Case III}$: $(4m-8n+5)=7$ and $(4m+8n-5)=33$ $\Rightarrow$ $m=5$ and $n=-1$, $|m-n|=6$.
  • $\textbf{Case IV}$: $(4m-8n+5)=11$ and $(4m+8n-5)=21$ $\Rightarrow$ $m=4$ and $n=0$, $|m-n|=4$.

Other cases in which the value of $(4m-8n+5)$ and $(4m+8n-5)$ interchange, the values of m and n will not change in those cases. Thus the maximum value of $|m-n|=\boxed{14}$.

~Lakshya Pamecha and Parveen Sir