Equation of a rectangle I need to graph a rectangle on the Cartesian coordinate system. Is there an equation for a rectangle? I can't find it anywhere. 

If the equations of the diagonals of the rectangle are Ax + By + C = 0 and Dx + Ey + F = 0 then an equation for the rectangle is:
M|Ax + By + C| + N|Dx + Ey + F| = 1
M and N can be found by substituting the coordinates of two adjacent vertices of the rectangle.
In fact, this equation can be used to describe any parallelogram.  Roughly speaking, M (together with A and B) and N (together with D and E) give the size of the diagonals of the parallelogram. 
