Tangencies

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1. One straight line r is tangent to

a circumference when they have a single

point of contact M and the straight line

is perpendicular to the its radius at M.

2. Two circumferences are tangent

when the straight line joining their

vertices passes through their point

of tangency M.

3. A circumference is tangent to two

convergent lines when its center is

on their angle bisector and has one

point of tangency with each line.