Circles: Special Segments in Circles

Tangents to Two Circles

Lines might be tangent to two separate circles at the same time. The line is either an external tangent or an internal tangent.

External Tangent:
An external tangent is on the outside of both circles. An example of this would be a bicycle chain connecting two gears.

Two circles with a line tangent to both circles on the same side
External Tangent

Internal Tangent:
An internal tangent is on the inside of the circles. An example of this would be a pulley.

Two circles with a line tangent to both circles on the opposite sides
Internal Tangent

Internal tangent pulley system

At this point, we will not go into detail about the properties of these tangent lines, but it is important to know the difference between the two. This is a topic that will be covered in depth in an upper level geometry course. When you get to those, you will be familiar with the tangents they are referring to.