Here are some Internet sources which may be helpful.

http://illuminations.nctm.org/LessonDetail.aspx?ID=L703

http://www.clarku.edu/~djoyce/trig/

http://mathworld.wolfram.com/Trigonometry.html

http://www.sosmath.com/trig/trig.html

http://en.wikipedia.org/wiki/Trigonometry

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