Part of Eric Weisstein's comprehensive math site, this page deals with double-angle formulas. This site includes traditional trigonomic double-angle functions as well as hyperbolic trigonometric function. Includes links to related terms...

This site from MathWorld explains the hyperbolic function. The information is not too in-depth, but factual and quite a few links are provided for additional information on related subjects.

This lesson is the main SOS Math page for trigonometry. Surf the lesson to find most any trigonometric identity, formula, and many forms of help in the area of trigonometry.