In that case, it means both the positive and negative square roots of x . Variables Most variables symbols will be single letters. Some examples are a, b, i, j, t, u, v, x, y . Sometimes I will use upper case letters as well for variable symbols. Often the upper case letters will be used to represent constants.
Variables can represent integers, real numbers, constants, or functions.
When the independent variable of a function needs to be shown, it will follow the symbol for the function immediately, but in parentheses. Some variables in calculus are traditionally shown using Greek letters. Since I can't make these symbols over the web, I will spell out Greek letters, such as alpha, beta, epsilon, theta, phi . This notation can be extended to any number of dimensions. That means it is the set of all ordered pairs that you can make by taking the first element of the ordered pair from A and the second from B .