Proof that middle Points of Diagonals of Quadrilateral lie in a Right Line . Intersections of Perpendiculars, of Bisectors of Sides, and of Perpendiculars at middle Points of Sides, lie in a Right Line . .

Angle between two Lines given by a single Equation . Equation of Bisectors of Angles between these Lines Condition that Equation of second Degree should represent Right Lines (see also :

Equation of Circle . Conditions that general Equation may represent a circle Coordinates of Centre and Radius . Condition that two Circles may be concentric. .