https://www.mathsisfun.com/triangle.html
http://www.coolmath.com/reference/triangles-geometry
http://www.sparknotes.com/testprep/books/newsat/powertactics/geometry/chapter2section2.rhtml
http://www.coolmath.com/reference/triangles-geometry
http://www.sparknotes.com/testprep/books/newsat/powertactics/geometry/chapter2section2.rhtml