# Results and Formulas of a Circle

**01. **The equation of a circle having the center at and radius is

**02.** The equation of a circle having the center and radius is

**03.** The equation of a circle in general form is , whose center is at and radius is

**04.** The equation of a circle passing through the point of intersection of the circles and is .

**05.** If and are the equations of two intersecting circles, then is the equation of the common chord.

**06.** If and are the equations of two circles such that they touch each other, then is the equation of the common tangent.

**07.** If and are the equations of two non-intersecting circles, then is the equation of the radical axis.

**08.** The length of the tangent segment from a point to the circle is given by

**09.** A system of circles coaxal with the circles and is