by Brad Simmons
Tangent from a Point Outside the Circle
Use geometer's sketchpad to construct a tangent line from a point outside the circle.
Construct a line tangent to a given circle C through a point A outside the circle.
1. Construct segment AC.
3. Construct the midpoint of segment AC. Label the midpoint M.
4. Construct segment CM. Select midpoint M and segment CM. Construct circle by center and radius.
5. Label the intersections of the two circles point D and point E.
6. Select circle M. Select segment CM and segment AC. Select midpoint M. Hide the objects. Construct line AD.
Line AD is tangent to circle C. Furthermore, line AE is also tangent to circle C.
Please click here for a geometer's sketchpad sketch of the figure shown above.
Please click here for more information about constructing common tangents to circles.