What does the comma notation mean? I am confused by the notation comma.
I know that the comma means 'AND' in Set theory as gate ($a \land b=a$ AND $b$), 
But we write solution of equation as $x=1$, $2$ (the equation: $x^2-3x+2=0$)
The question is whether $x=1,2$ is WRONG?
$x=1,2$ $\iff$ $x=1$ AND $x=2$, so rewritten as $x=1$ or $x=2$.
But in so many books, it is written as $x=1$, $2$.
I am confused.....
(PS. I think the comma of $x=1$, $2$ is only notation of classification...? Is it ok?)

First of all you should know, that there are more countries where the comma is a decimal separator than there are point-separator-countries. (For example: I live in Austria in Europe, and we use the comma as decimal separator.) The international standard since about 100 years is to use a point as decimal separator (before that time the comma was the international decimal separator).

blue: decimal separator is a point ($\pi = 3.14$)
green:  decimal separator is a comma ($\pi = 3$,$14$)
red: decimal separator is a momayyez ($\pi = 3٫14$)
other colors: two or all three of the above standards are in use 
In Countries where the comma is not used as decimal separator (not-green countries in the picture), it is used as list-separator, for example when you want to list the elements of a set. This is also used internationally:

set = {Apple, Zwetschke, 42, -47.6, 小数点}

In countries where the decimal-separator is a comma (green countries), the semicolon is used als list-separator:

set = {Apple; Zwetschke; 42; -47,6; 小数点}

Often a mathematical problem has more than one solutions (for example $x^2-5x+6$). So the solution is a set of numbers:

Solution = {$x=2$, $x=3$}

But a shorter way to write the same fact is this:

$x = 1, 2$ in point-countries
  $x = 1; 2$ in comma-countries  

