12.1 Tangents

When each side of a polygon is tangent to a circle, the polygon is said to be circumscribed about the circle, the circle is inscribed in the polygon.

Common tangent  a line that is tangent to each of 2 coplanar circles.

Common internal tangent a common tangent that intersects the segment joining the centers of the 2 circles.

Common external tangent  a common tangent that does not intersect the segment joining the centers.

A circle can also be tangent to another circle. Tangent circles are coplanar circles that are tangent to the same line at the same point.