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The solutions of an equation in one variable form a set of numbers; the solutions of an equation in two variables form a set of ordered pairs of numbers, which can be plotted in the coordinate plane. Two or more equations and/or inequalities form a system. A solution for such a system must satisfy every equation and inequality in the system.

An equation can often be solved by successively deducing from it one or more simpler equations. For example, one can add the same constant to both sides without changing the solutions, but squaring both sides might lead to extraneous solutions. Strategic competence in solving includes looking ahead for productive manipulations and anticipating the nature and number of solutions.

Some equations have no solutions in a given number system, but have a solution in a larger system. For example, the solution of 𝑥 + 1 = 0 is an integer, not a whole number; the solution of 2𝑥 + 1 = 0 is a rational number, not an integer; the solutions of 𝑥² – 2 = 0 are real numbers, not rational numbers; and the solutions of 𝑥² + 2 = 0 are complex numbers, not real numbers.