A common form of a linear equation in the two variables x and y is
where m and b designate constants (parameters). The
origin of the name "linear" comes from the fact that the set of
solutions of such an equation forms a straight line in the plane. In this particular equation, the constant m determines the slope or gradient of that line, and the constant term b determines the point at which the line crosses the y-axis, otherwise known as the y-intercept.
Since terms of linear equations cannot contain products of distinct
or equal variables, nor any power (other than 1) or other function of a
variable, equations involving terms such as xy, x2, y1/3, and sin(x) are nonlinear.
Forms for 2D linear equations
Linear equations can be rewritten using the laws of elementary algebra into several different forms. These equations are often referred to as the "equations of the straight line." In what follows, x, y, t, and θ are variables; other letters represent constants (fixed numbers).