Massachusetts Institute of Technology
Mit: Useful Trigonometric Identities
This list of trigonometric identities includes the factoring formulas near the bottom. A great tool for reference.
Wolfram Research
Wolfram Math World: Half Angle Formulas
This page, from Eric Weisstein's comprehensive Math World site, offers information on half-angle formulas. The site includes traditional trigonometric fuctions half-angle formulas as well as hyperbolic half-angle formulas. Includes links...
Math Medics
S.o.s. Math: Double Angle and Half Angle Formulas
Double angles and half angles are explained through step by step examples. The steps are clear and easy to follow.
CK-12 Foundation
Ck 12: Trigonometric Equations Using Half Angle Formulas
[Free Registration/Login may be required to access all resource tools.] Here you'll learn how to solve trigonometric equations using the half angle formulas.
Lawrence Hall of Science
The Math Page: Trigonometry: Trigonometric Identities
This resource clearly and thoroughly explains the many types of trigonometric identities. There are example problems solved, problems for the student to attempt, and answers to the student problems.
CK-12 Foundation
Ck 12: Trigonometry: Half Angle Formulas
[Free Registration/Login may be required to access all resource tools.] Using identities to derive more half angle formulas.
eFunda
E Funda: Multiple Angle
At this site from eFunda you can scroll to the middle of the page for a listing of multiple angle formulas for cos, sin, and tan.