A line can be determined by two points on the line or by a single point with the direction of the line. In two dimensions the direction can be specified by the gradient. There are a number of related standard formats for lines in a plane.
Slope‐intercept form.
y = mx + c. The line has a gradient m and intercept at (0,c). For a vertical line the gradient is infinite and the equation is x = k (in this case there is no intercept on the y-axis).
Two‐point form.
The condition that the general point P(x,y) lies on the line through A(x1,y1) and B(x2,y2) is equivalent to requiring the gradient of AP = the gradient of
Point‐slope form.
A line with gradient m passing through the point A(x1,y1) has equation y−y1 = m(x−x1).
Two‐intercept form.
When the equation is written as the line cuts the axes at (a,0) and (0,b), making it very easy to draw the line.