Math Open Reference is a great supplement to mathematics textbooks and in particular for learners who need visual references.

 

Math Open Reference...
     - is a free online reference for geometry practitioners and learners.
     - features animated and interactive drawings to demonstrate geometry terms and concepts.

 

By clicking on any subject in the first three categories users will find definitions, examples, and interactive drawings.

 

In the function explorer category users can select linear functions, quadratic functions, or cubic functions to explore how changes in variables affect the graphed output.