A constraint is a limitation on the possible values of variables in a model. It is often expressed by an equation or inequality or by specifying that the value must be an integer.

For example, distance above the ground \(d\), in meters, might be constrained to be non-negative, expressed by \(d \ge 0\).

Elimination is a method of solving a system of two equations in two variables. A multiple of one equation is added to or subtracted from another to get an equation with only one of the variables. (The other variable is eliminated.)