PRINCIPIA · THEOREM
Two tangents from an external point have equal length
Depends on: Tangent is perpendicular to the radius at the point of tangency, HL (hypotenuse-leg) congruence.
Statement
Let be the circle of centre and radius , and let be a point outside . From draw two tangents to , touching at and . Then the two tangent lengths are equal:
Furthermore, the segment joining the centre to bisects both symmetric angles:
That is, is simultaneously the Angle bisector ⇔ equidistant from sides of and of .

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