Definition:External Tangent
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Definition
Let $C_1$ and $C_2$ be circles embedded in the plane such that:
- $C_1$ and $C_2$ do not intersect
- one is not inside the other.
Consider the $4$ common tangents to $C_1$ and $C_2$.
The external tangents are the $2$ common tangents which have the circles on the same side of the tangent.
In the above diagram, the $\color { blue } {\text {blue} }$ lines are the external tangents.
Also see
- Two Non-Intersecting Circles have Four Common Tangents: a proof that there are $4$ such common tangents
- Results about common tangents can be found here.
Sources
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): common tangent
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): external tangent
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): common tangent
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): external tangent