Defining trigonometric ratios from the unit circle. See also:
Unit Circle and Sine Graph: http://www.geogebratube.org/material/show/id/8028
Unit Circle and Cosine Graph: http://www.geogebratube.org/material/show/id/8029
Unit Circle and Tangent Graph: http://www.geogebratube.org/material/show/id/8030
Anthony Or. GeoGebra Institute of Hong Kong.
E-mail: orchiming@gmail.com