Purple Math
Purplemath: Slope of a Straight Line
This lesson on slope explains the slope concepts, demonstrates how to use the slope formula, shows how to find slope in horizontal and vertical lines, and how to use slope to find whether the lines through the pairs of points are...
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Equation for Undefined Slope
This site gives the equation for a vertical line which passes through 8 on the x axis. The Dr. explains why the equation is what it is by giving a more general equation for a line than y = mx + b.