Find Integer Solutions Problem

1. Apr 3, 2005

Andromeda321

Hey everyone,
I came accross this problem recently and I'm trying to find an answer for it to satisfy my curiousity (that and it's easy to understand but hard to actually solve, so tantalizing!). Can anyone give me a nudge in the right direction?

Find all ordered paris that are integer solutions to the following equation:
xy/ (x+y)= 4

Thanks!

2. Apr 3, 2005

Moo Of Doom

Solving for x in terms of y, or y in terms of x will help. Then find what integer values for one will make the other an integer.

3. Apr 3, 2005

Andromeda321

I tried that but couldn't get one completely separated from the other. For example when you solve for x you get 4(x+y)/y which doesn't really help you much.

4. Apr 3, 2005

Jameson

You can write this as $$xy = 4x + 4y$$ , but I don't believe you can simply "look" for the solution. That is entirely unmathematical.

5. Apr 3, 2005

Moo Of Doom

Alright, so $$\frac{xy}{x+y}=4$$ which means $$xy=4(x+y)=4x+4y$$
$$xy-4x=4y$$
$$x(y-4)=4y$$
$$x=\frac{4y}{y-4}$$

For what values of y is 4y divisible by y-4?

6. Apr 3, 2005

tongos

it would be easier to put it in this form, then do it...

y-4=C 4(C+4)/C C>4

(4C+4)/C= 4+(16/C) so now we figure that 16/C . so the only values is the factors of 16 excluding all those less than 4. this is a big hint.

7. Apr 3, 2005

Andromeda321

*smacks head* Ok, got it now! Nine values all told, and then I computed limits to show that there were no other values it could possibly be before (-12, 3) and after (20, 5). Thanks guys!

8. Apr 23, 2005

abia ubong

i feel u should equate xy/[x+y]as 4n/nwhich is still the samenow in this caseu might take xy 2 be 4n and x+y as n .then solve the simul eqn.n being any number,just give it a try