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2016-03-18T20:29:31+05:30
Tangent is perpendicular to radius at point of contact
tangent drawn from external point are equal

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2016-03-18T20:29:50+05:30
Central angles and arcs:1.    If two central angles of a circle (or of congruent circles) are congruent, then their intercepted arcs are congruent. (Short form: If central angles congruent, then arcs congruent.)2.    If two arcs of a circle (or of congruent circles) are congruent, then the corresponding central angles are congruent. (Short form: If arcs congruent, then central angles congruent.)Central angles and chords:3.    If two central angles of a circle (or of congruent circles) are congruent, then the corresponding chords are congruent. (Short form: If central angles congruent, then chords congruent.)4.    If two chords of a circle (or of congruent circles) are congruent, then the corresponding central angles are congruent. (Short form: If chords congruent, then central angles congruent.)Arcs and chords:5.    If two arcs of a circle (or of congruent circles) are congruent, then the corresponding chords are congruent. (Short form: If arcs congruent, then chords congruent.6.    If two chords of a circle (or of congruent circles) are congruent, then the corresponding arcs are congruent. (Short form: If chords congruent, then arcs congruent.)

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http://www.dummies.com/how-to/content/six-important-circle-theorems.html
you can consult this site for more info.