0

Thanks by your help I found a solution and I will leave it here if someone finds this problem interesting

$$\begin{cases}ab+1&=&(c+1)(d+1)\\ cd+1&=&(a-1)(b-1)\end{cases}$$

When you expand the both sides of the equations you get: $$\begin{cases}ab+1&=&cd+c+d+1\\ cd+1&=&ab-a-b+1\end{cases}$$

Sum both equations and you get: $$\begin{cases}a+b&=&c+d\\ ab-cd&=&a+b\end{cases}$$

From here it’s obvious that

ab-cd=a+b=c+d and let’s consider these sums equal to s

Then

s=a(s-a)-c(s-c)=(a-c)(s-a-c)

If s is odd then a-c & s-a-c are also odd but then their sum s-2c would be even and so s would be even too

Therefore a-c and s-a-c are even & let’s consider a-c=2x and s-a-c=2y where x,y are integers

From this we get that s=4xy and then we’re done

We get (a,b,c,d)=(2xy+x-y,2xy-x+y,2xy-x-y,2xy+x+y)

  • 1
    Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be closed. To prevent that, please [edit] the question. This will help you recognize and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers. – José Carlos Santos Sep 05 '23 at 09:01
  • When you expand it and sum both equations you get a+b=c+d and then idk what to do. One obvious solution is 0;0;0;0 – Jar Jar Binks Sep 05 '23 at 09:08
  • The comments section is not the place to add your work. – jjagmath Sep 05 '23 at 09:18
  • oh okay. I don’t really know how to work with this app but I’ll try – Jar Jar Binks Sep 05 '23 at 09:23
  • The system has infinitely many solutions. To find them, think $b$ and $c$ as given constants and solve for $a$ and $d$. You'll end with a parametrization of all the solutions. – jjagmath Sep 05 '23 at 09:52
  • I can only find c=(a+b)/2 How do you suggest I find the other parameters – Jar Jar Binks Sep 05 '23 at 10:06
  • If you think $b$ and $c$ as constants you have a system of two linear equations ($a+b = c+d$ and $cd=ab-a-b$) in two unknowns ($a$ and $d$). Do you know how to solve a system of two linear equations in two unknowns? – jjagmath Sep 05 '23 at 15:09
  • Yeah I found a solution. Should I delete my post after I got what I needed? Thanks for the help all of you guys are amazing – Jar Jar Binks Sep 05 '23 at 18:39

0 Answers0