Find all integers $x,y$ such that $2x+3y=185$ and $xy>x+y$.
Problem
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Tags: number theory
Richie
08.04.2021 08:32
I may be wrong but I am getting $29$ solutions of the equation.
Wildabandon
08.04.2021 09:23
jasperE3 wrote: Find all integers $x,y$ such that $2x+3y=185$ and $xy>x+y$.
From $2x+3y=185$, we have $(x,y) = (-185+3k,185+2k)$ where $k\in \mathbb{Z}$. Because $xy>x+y$, we have $6k^2 + 180k -185^2>0$. Then,
\[k > \frac{5\sqrt{8538}- 90}{6} \quad \text{or}\quad k<\frac{-5\sqrt{8538} - 90}{6}.\]Because $k\in \mathbb{Z}$, =we conclude that $k\ge 62$ or $k\le -93$. So, the solutions are
\[\boxed{(x,y) = (-185+3k,185+2k), \; k\in \mathbb{Z},\; k\in (-\infty,-93]\cup [62,\infty)}.\]
cosmicc
23.05.2021 12:13
Wildabandon wrote: jasperE3 wrote: Find all integers $x,y$ such that $2x+3y=185$ and $xy>x+y$.
From $2x+3y=185$, we have $(x,y) = (-185+3k,185+2k)$ where $k\in \mathbb{Z}$. Because $xy>x+y$, we have $6k^2 + 180k -185^2>0$. Then,
\[k > \frac{5\sqrt{8538}- 90}{6} \quad \text{or}\quad k<\frac{-5\sqrt{8538} - 90}{6}.\]Because $k\in \mathbb{Z}$, =we conclude that $k\ge 62$ or $k\le -93$. So, the solutions are
\[\boxed{(x,y) = (-185+3k,185+2k), \; k\in \mathbb{Z},\; k\in (-\infty,-93]\cup [62,\infty)}.\]
62 for k doesn't work
Pal702004
24.05.2021 18:06
$2x+3y=185$
Integer solutions:
$y=1+2k,x=91-3k,\;\;k \in \mathbb{Z}$
$xy>x+y \; \Leftrightarrow (x-1)(y-1)>1$
$(90-3k)(2k)>1 \; \Rightarrow (30-k)k>0$
$1\le k\le 29$