Varsity Tutors
Varsity Tutors: Hotmath: Graphing Calculator Practice Problems: Proportions
Practice using your graphing calculator for solving proportion problems. If you get stumped or if you would like to check your work, then select the View Solution button, which is located directly under each problem.
University of Illinois
University of Illinois: Quadratic Formula
This site from the University of Illinois offers an excellent simulation of the quadratic formula, where you can type in different values for a, b, and c, and then view the change in the answer for x.