often sets consist of those elements satisfying some condition or possessing some specifed property. In these case, we can define such a set as S we mean that S consists of all those elements x satisfying some condition p(x) concerning Some mathematicians write S (r I p(x)); that is, some prefer to write a vertical line rather than colon (which, by itself here, is understood to mean "such that''). For example, if we are studying real number solutions of equations, then is the set of all real number x such that (x-1) that is, s is the solution set of the equation (r -1)(r 2)(x 3) 0. We could have written S (I. -2, -3): however, even though this way of expressing s is apparently simpler, it does not tell us that we are interested in the solutions of an equation. The absolute value lxl of a real number x is x x 0; while lurl r if r 0. Therefore, if is the set of all real numbers having absolutdrvalue 2; is, T (2, -2). In the sets S and T that we have just described, we understand that "r" refers to a real number x