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.
Wolfram Research
Wolfram Math World: Tangent
Graphs of the tangent function, both 2-dimensional and 3-dimensional. Higher level mathematics applications given including the hyperbolic tangent function, important relevant identities, the Macluarin series for the tangent function,...