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: Inverse Cosine
A site that deals with inverse cosine. Article includes the definition, formulas including the Maclaurin series for the inverse cosine function, and examples, as well as numerous links to related terms.
Clark University
Clark University: Dave's Short Trig Course
This Clark University trigonometry tutorial site features questions and answers about what students can learn from trigonometry.
Other
Technical Tutoring: Trigonometry: The Very Fast Review
This site is a quick review of the functions and identities in trigonometry. Very helpful site.
Other
Lifewire: Excel Tan Function: How to Find the Tangent Angle
This article shows you how to find the tangent angle using Excel. You may be able to accomplish this on a scientific calculator, but if you don't have one handy, Excel can help you make these calculations better than anything else.