Mathematics Asked by Estagon on August 28, 2020
The equation is as follows:
$$y=frac {c-x}{x+1}$$
Where $c$ is a positive, even integer.
Is it possible to determine whether there are any positive integer solutions different than $x=0$?
Some examples:
$c=14$; $x=2$; $y=4$.
$c=10$; NO POSITIVE INTEGER SOLUTION
$c=20$; $x=2$; $y=6$.
$c=44$; $x=4$; $y=8$.
$c=76$; $x=6$; $y=10$.
Hint: You have
$$y = frac{c - x}{x + 1} = frac{c + 1 - 1 - x}{x + 1} = frac{c + 1 - (1 + x)}{x + 1} = frac{c + 1}{x + 1} - 1$$
Next, consider how whether or not $c + 1$ is prime, e.g., where $c = 10$, affects the existence of positive integer solutions for $x$ and $y$.
Correct answer by John Omielan on August 28, 2020
It was not perfectly clear to me whether this was a question about suitable $x$ (any solutions other than $x=0$?) or about suitable $c$ (listing various $c$ for which $x,y$ could be found). I provide an answer about $x$, and the implications for $c$.
$y$ is an integer when $c=k(x+1)+x$, viz: $y=frac{k(x+1)+x-x}{x+1}=k$.
However, since $x$ and $x+1$ have different parities, $c$ cannot be even when $x$ is odd. When $x$ is even, $k=2n$ must be even for $c=2n(x+1)+x$ to be even. In that case, $y=frac{2n(x+1)+x-x}{x+1}=2n$.
In sum, solutions exist for all $x$ and $y$ being any even integers; $c$ is limited to being of the form $c=2n(x+1)+x$ for any particular chosen $x$.
Answered by Keith Backman on August 28, 2020
A slightly different wording than John Omielan's answer (but same idea) may make this more intuitively clearer.
$y = frac {c-x}{x+1}$ will have solutions when $x+1|c-x$.
And $x+1|c-x iff x+1|(c-x)+(x+1)= c+1$.
So for any $k|d=c+1$, we will have solutions $x = k-1$ (and $y = frac {c-(k-1)}{(k-1)+1}=frac {(c+1)-k}k = frac dk -1$)
However if $d=c+1$ is prime we either $x= 0$ or $x = d-1=c$ (in which case $y=0$).
Answered by fleablood on August 28, 2020
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