http://www.mathsisfun.com/sine-cosine-tangent.html
http://www.slidermath.com/rpoly/Trigapps.shtml
http://m.youtube.com/watch?v=nfMkORv6ybc&safety_mode=true&persist_safety_mode=1&safe=active
http://www.s-cool.co.uk/gcse/maths/trigonometry/revise-it/sin-cos-tan
http://www.khanacademy.org/math/trigonometry/v/basic-trigonometry
http://www.khanacademy.org/math/trigonometry/v/basic-trigonometry-ii