Geometry - the study of shape

The Circle (Simple Wiki).: Measurement of a Circle: a treatise of three propositions by Archimedes.; Equation of a Circle; More on Equation of a Circle; Even more on Equation of a Circle
Circle Theorems: Inscribed Angle Theorem; Intersecting Chords Theorem; If $(x_{1}, y_{1})$ and $(x_{2}, y_{2})$ are the co-ordinates of the end of the diameter of a circle, then $(x - x_{1}) (x - x_{2}) + (y - y_{1}) (y - y_{2}) = 0$