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Tangent lines have been defined and studied earlier in this course. In this lesson, methods for constructing a tangent line to a circle from an external point will be discussed.

Catch-Up and Review

Here is some recommended reading before getting started with this lesson.


Investigating Properties of Tangents to a Circle

Consider a circle with center and point outside of the circle. Using a straightedge and compass, can you construct a tangent to the circle through the given point?

Circle and External Point
Think about the properties of tangents. How can you justify that the line you draw is indeed the tangent?


Drawing a Tangent Line to a Circle Through an Outer Point

Given a circle and a point outside the circle, a compass and straightedge can be used to draw a tangent line from the point to the circle.

There are four steps to construct a tangent line.
Using a straightedge, draw the segment connecting the point and the center of the circle
Mark Midpoint of
To locate the midpoint of the perpendicular bisector of can be used. Recall that there are three steps to draw a perpendicular bisector.
The point where and its perpendicular bisector intersect is the midpoint of
Draw a Circle with Center and Radius
Next, place the compass tip on and stretch the compass to or Then, draw the circle
Draw Tangents
The points where the circles intersect will be the points of tangency for the tangents drawn from to the circle

The Inscribed Right Triangle Theorem can be used to justify why this construction works.


Consider a radius of

In circle is an inscribed angle on a diameter of Since an inscribed angle opposite the diameter is a right angle, is a right angle.

As can bee seen, The radius of is perpendicular to a line that passes through a point on the circle. Therefore, by the Tangent to Circle Theorem, is tangent to the circle.


Using Tangents to a Circle to Construct an Eye

Kriz is learning a graphic program. By default, the program shows segment and circle The segment's endpoint can be moved anywhere outside of While endpoint can be moved anywhere. An eye-like shape appears on the screen when is tangent to the circle. Give it a try!

Kriz can't quite place point in position to see the eye-like shape appear. Help Kriz out!


See solution.


How can a tangent line from a point outside of the given circle be constructed?


Since point is a point outside should be the point of tangency in order for to be tangent to the circle. On the example shape, by extending it can be observed that is the point of tangency.

Constructing a tangent from an outer point will help locate the point of tangency for a tangent drawn from Recall the steps in constructing a tangent.

  1. Connect the outer point and the center of the circle.
  2. Find midpoint of the segment drawn.
  3. Draw a circle with center and radius half the segment drawn.
  4. Draw tangent.

In this case, point is the outer point through which the tangent line is drawn. To get the example shape, move point A to the left as shown and then follow the steps.

As can be seen, the points where the circles intersect are the points of tangency. Therefore, point should be on these points.


Relationship Between Tangents and Lines of Symmetry

The lines of symmetry of a circle are the lines that passes through the center of the circle.

Suppose that a tangent line drawn from an outer point intersects a circle at When is reflected across the line that passes through and its image will also be a point of tangency for another tangent.


External Tangent Congruence Theorem

Two tangent segments drawn from a common external point to the same circle are congruent.

For the above diagram, the following conditional statement holds true.

If and are tangent segments to then


Consider two triangles.

  • The triangle formed by the radius the segment and the tangent segment
  • The triangle formed by the radius the segment and the tangent segment

These two triangles can be visualized in the diagram.

Note that and are points of tangency. Therefore, by the Tangent to Circle Theorem, and are right angles. Consequently, and are right triangles.

Because all radii of the same circle are congruent, it can be said that and are congruent. Moreover, and share the same hypotenuse By the Reflexive Property of Congruence, is congruent to itself.

Combining all of this information, it can be said that the hypotenuse and one leg of are congruent to the hypotenuse and the corresponding leg of

Therefore, by the Hypotenuse-Leg Theorem, and are congruent triangles. Since corresponding parts of congruent figures are congruent, it can be said that and are congruent.


External Tangent Congruence Theorem

In the diagram, all three segments are tangent to circle

The points and are the points where the segments touch the circle. If and find


From the graph, it can be seen that and are tangent segments with a common endpoint outside By the External Tangent Congruence Theorem, and are congruent.

Smilarly, and and and are congruent tangent segments.
Using the Segment Addition Postulate and the given lengths, a system of equations with three equations and three unknowns can be written.
To find the Elimination Method can be used. Start by multiplying the second equation by
Adding this equation to the first equation will eliminate
Since and are additive inverses, adding this equation to the third equation will eliminate and thus give


Tangents to Circles in Real Life

Imagine a superhero joining the Olympics to throw a hammer. An athlete would typically spin counterclockwise three or four (rarely five) times, then release the hammer. As viewed from above, the hammer travels on a path that is tangent to the circle created when the athlete spins. The diagram below shows the path of the superhero's hammer throw. See how the super hero fairs! Note, it is a not-to-scale drawing.
On August in a packed stadium full of fans, Yuriy Sedykh set the world record with a throw of meters. Perhaps a throw farther than the superhero!
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