1. Jan 10, 2009

### Peter159

How can i find all integers to this equation?

3(x+y)-xy=0

I allready found just by tring that if x=y then its 6 and x=12 ,y=4 (x=4 and y=12)

I think that there are more, but how can i find all of them?

PS. the first task was: 1/x + 1/y = 1/3 and then i had to find all integers.

2. Jan 10, 2009

### Dick

I would write it in the form x=3*y/(y-3). For large values of y thats a little larger than 3 and not an integer. So once you tried a y value large enough that 3*y/(y-3)<4, there aren't any larger solutions. Then I'd just try all y values less than that to see if x is also an integer. Do you need negative solutions as well?

3. Jan 10, 2009

### snipez90

$$\frac{1}{x} + \frac{1}{y} = \frac{1}{3} \Rightarrow 3x + 3y = xy \Rightarrow xy - 3x - 3y + 9 = 9 \Rightarrow (x-3)(y-3) = 9$$.

The number 9 has three positive divisors: 1, 3, and 9. You just have to solve each of the systems of equations to get all the solutions.

4. Jan 10, 2009

### Dick

Very clever.

5. Jan 11, 2009

### Peter159

Ok, thanks Dick and Snipez90 for your help!

I have one question. xy-3x-3y+9=9 > (x-3)(y-3) How did you factor that one?