Nature and number of solutions to $xy=x+y$ 
Find all solutions to $$xy=x+y$$ Initially the given condition was $x,y\in \Bbb{Z}$. 

$$$$In this case, I just guessed that the solutions were $(0,0)$ and $(2,2)$. As far as I can see, these are the only 2 integral solutions possible. However, i'm quite surprised as usually a Diophantine Equation has infinite solutions. Could somebody please show me how to actually $solve$ this equation instead of just guessing the values? Is there any way to $show$ that there are just 2 sets of solutions?
$$$$Secondly, what if $x,y\in \Bbb{R}$? In that case how could the equation be solved?
$$$$For both these conditions on the values of $x,y$, is there any way to use Coordinate Geometry to achieve an answer?
$$$$
Many thanks in anticipation!
Edit: I forgot to mention that I know the solution using factoring. I was hoping to find a solution using Coordinate Geometry. I was told by a friend that the non integral solutions lie on 2 unique lines, and am hence particularly interested in a geometrical solution.
 A: Of course the simplest solution is to write the equation as $(x-1)(y-1)=1$.

The curve $xy-x-y=0$ is a hyperbola with asymptotes $x=1$ and $y=1$.
Its graph is so contained in the union of the strips
$$
(-\infty,0]\times[0,1),
\qquad
[0,1)\times(-\infty,0],
\qquad
(1,2]\times[2,\infty),
\qquad
[2,\infty)\times(1,2]
$$

The only points with both integer coordinates are $(0,0)$ and $(2,2)$.
More analytically, consider the curve written as
$$
y=1+\frac{1}{x-1}=\frac{-x}{1-x}
$$
If $x>2$, then $x-1>1$, so $1<y<2$, so $y$ is not integer. If $x<0$, then $0<y<1$, so $y$ is not integer. For $0<x<1$ and $1<x<2$, $x$ is not integer.
Thus only $x=0$ or $x=2$ remain.
A: You can say: 
$x = u+v\\
y = u-v\\$
plug it into your equation and you get:
$u^2 - v^2 = 2u\\
(u-1)^2 - v^2 = 1$
And you should recognize that as a hyperbola.  And have a idea how to get solutions in terms of $(u,v)$.  And then use the substitution again to get those solutions in terms of $(x,y)$.
What have I done?  I have changed the coordinate system such that the line $y=x$ is my new "$u-$axis" and $y = -x$ is my new "$v-$axis."  That is, I have rotated the system 45 degrees.
Now someone is going to give me grief (unless I am able to dispel it....)
The change that I made does not preserve distance.  So, everything is a little bit compressed in the new coordinate system.
To which I say, is that such a big deal?
If I were a good boy, I would have said:
$u = x \cos \frac\pi4 - y\sin \frac\pi4 \\
v = x \sin \frac\pi4 + y\cos \frac\pi4 $
$x = \frac {\sqrt 2}{2} (u+v)\\
y = \frac {\sqrt 2}{2} (v-u)$
Swapping my u's and v's.
$v^2 - u^2 = 2\sqrt 2 v\\
(v- \sqrt 2)^2 - u^2 = 2$
Giving a very similar equation.
A: To solve $x+y=xy$ in integers, note that it implies that $x\mid y$ and $y\mid x$.  Therefore, $y=\pm x$.  Can you show from there that $(0,0)$ and $(2,2)$ are the only integer solutions?
A: Clearly, $a\ne1.$ Solving for $b$:
$$b={a\over a-1}\tag1$$
If $a=0.$ then (1) gives the solution $(a.b)=(0,0).$
Otherwise, the numerator and denominator in (1) are
coprime, implying that $a=1\pm1,$ giving exactly one more solution, namely $(2,2).$
A: In the case that$(x,y) \in \Bbb{Q}$,
 setting 
$x=\frac{2}{1-u}$ and $y=\frac{2}{1+u}$ then
$x+y=\frac{4}{1-u^{2}}$
$x y=\frac{4}{1-u^{2}}$
and 
if  $ 0<u<1$ then $ (x,y)>(0,0)$.
If $u=\frac{1}{2}$ then $ (x,y)>(4, 4/3)$ and so on…
