1. Section 9-2
Tangents

2. Theorem 9-1
If a line is tangent to a circle, then the line is perpendicular to the radius drawn to the point of tangency.

3. Point of Tangency

4. Corollary
Tangents to a circle from a common point are congruent A P B

5. Theorem 9-2
If a line in the plane of a circle is perpendicular to the radius at its outer endpoint, then the line is tangent to the circle.

6. Tangent line

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

8. Common tangent
A line that is tangent to each of two coplanar circles

9. Common internal tangent
Intersects the segment joining the centers A B

10. Common external tangent
Does not intersect the segment joining the centers. A B

11. Tangent circles
Coplanar circles that are tangent to the same line at the same point

12. Externally Tangent l A B

13. Internally Tangent l C D