Question

How to write a domain

Answer 1

The domain are all valid values for x (the independent variable) that can be used in an equation.

We have to look at any potential values of x which won't work. Easily put: in algebra, just look for values of x which cause either division by zero, or the square root of negative numbers.

A couple of examples: 
y=2x+4
You can insert any negative or positive value, or zero, for x and get a valid equation. Therefore the domain is the set of all real numbers. Answers are usually written as:
x: {R}, or simply 'all real numbers'.

what about y=2/(x-1)
In this equation, x appears in the denominator. If x-1=0, then division by zero would occur. 
Solve: x-1≠0
x≠1
In set notation:
x: (-∞,1)∪(1,∞)
Parentheses are next to the 1, as the domain comes up to 1, but does not include 1. 
Read left to right, the domain is "negative infinity to 1, exclusive, in union with 1 to positive infinity"