Applications of lines include the use of equations or graphs of lines to model real-world situations and natural phenomena in order to make predictions and solve problems. Situations with a constant rate of change can be described by linear equations. A linear equation can be written in slope-intercept form as
is the slope, or constant rate of change, and
-intercept. The equation is useful for graphing the line and for interpreting the meaning of the slope and
-intercept in the context of a real-world application. A more simplified form of the slope-intercept form is
, where the
-intercept is zero. This is the general form of a direct variation relationship, which models situations that vary proportionally.