In mathematics, an equation is a formula of the form A = B, where A and B are expressions containing one or several variables called unknowns, and "=" denotes the equality binary relation. Although written in the form of proposition, an equation is not a statement, but a problem consisting in finding the values, called solutions, that, when substituted to the unknowns, yields equal values of expressions A and B. For example, 2 is the unique solution of the equation x + 2 = 4, in which the unknown is x. Historically, equations arose from the mathematical discipline of algebra, but later become ubiquitous. An equation may not be confused with identities which are presented with the same notation but have a different semantic: for example 2 + 2 = 4 and x + y = y + x are identities (which implies they are necessarily true) in arithmetic, and do not constitute any values-finding problem, even if include variables.