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5. Tangent Line to a Circle
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eCourses /
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Chapter 5
5. 

Tangent Line to a Circle

Tangent lines to circles have been a topic of interest and study in geometry. This lesson delves into the methods of constructing a tangent line to a circle from an external point. It emphasizes the properties of tangents and how to ascertain that a drawn line is indeed a tangent. The lesson further elaborates on drawing a tangent line through an outer point using a compass and straightedge. The process involves connecting the external point to the circle's center, marking the midpoint, drawing a circle with the midpoint as the center, and finally drawing the tangents. The lesson also touches upon the real-life application of tangents, illustrating it with an example of a hammer throw in the Olympics.
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11 Exercises - Grade E - A
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Tangent Line to a Circle
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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.

Challenge

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?
Discussion

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.
1
Draw
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Using a straightedge, draw the segment connecting the point and the center of the circle
2
Mark Midpoint of
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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
3
Draw a Circle with Center and Radius
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Next, place the compass tip on and stretch the compass to or Then, draw the circle
4
Draw Tangents
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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.

Why

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.

Example

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

Answer

See solution.

Hint

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

Solution

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.
Discussion

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.

Discussion

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

Proof


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.


Example

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

Hint

Use the External Tangent Congruence Theorem.

Solution

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
▼
Simplify
AddNeg

SubTerm

Subtract term

Since and are additive inverses, adding this equation to the third equation will eliminate and thus give
▼
Simplify
RemovePar

Remove parentheses

AddSubFrac

Add and subtract fractions

DivEqn
Closure

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!



Level 1
Level 2
Level 3
Tangent Line to a Circle
Exercise 2.1

In a factory, two gears are connected by a rubber band such that becomes a common tangent to both gears.

two gears connected with a rubber band
Calculate the distance between the gears' centers if is inches and the radii of the gears are and inches respectively. Answer in exact form.

Let's illustrate the situation. Notice that BC is tangent to both circles which means they are perpendicular to the radii of the circles. Let's label the segment between the gears' centers AD. Additionally, BA and CD are parallel segments.

Starting at point B, let's draw a segment to a point on CD in such a way that the segment is parallel to AD. This results in a right triangle with legs of 20 and 3 inches, respectively, and a parallelogram with two sides that are 1 inches long.

If we find the length of the segment that is parallel to AD, we also find the length of AD. To do that, we will use the Pythagorean Theorem.

The length of the parallel line, and therefore AD, is sqrt(409) inches.

Find the values of and

two circles with two common external tangents

According to the External Tangent Congruence Theorem, tangents drawn from a common external point are congruent segments. That means PA and PC are congruent. Let's show this relationship in the diagram.

Similarly, PB and PD are congruent. Notice that PB is the sum of PA and AB. At the same time, PD is the sum of PC and CD. Let's show these relationships in the diagram.

Now we can write two equations. &3x+3=y+8 & (3x+3)+(4y-29)=(y+8)+(x+6) If we combine these, we get a system of equations which we will solve by using the Substitution Method.

Having solved the system for x, we can substitute this into Equation (I) to calculate the value of y.

The line is tangent to the circles and What is the distance between the tangent points and if and have radii of and feet respectively and the length of is feet? Give the answer in exact form.

two circles P and O with a common tangent QR

Let's add all of the given information to the diagram. Notice that PQ and PA are both radii of ⊙ P. Also, OB and OR are both radii of ⊙ O.

By the Tangent to Circle Theorem, QR is perpendicular to both PQ and OR. To determine the length of QR, we can from O draw a parallel segment to QR, which intersects PQ. This segment will be congruent to QR.

Examining the diagram, we can spot a right triangle where the hypotenuse and the shorter leg are known.

Using the Pythagorean Theorem, we can find the measure of the unknown leg and thereby also QR.

The length of the horizontal leg is 3sqrt(165) feet. That is also the length of QR.

What is if is tangent to the circle?

Circle A with a segment drawn tangent to B on the circles circumference

We know that BP is tangent to ⊙ A. According to the Tangent to Circle Theorem, ∠ ABP is a right angle.

According to the Interior Angles Theorem, the angle measures of △ ABP add to 180^(∘). With this information, we can write and solve an equation containing the central angle ∠ BAP.

Notice that ∠ BAP and ∠ CAD are vertical angles. According to the Vertical Angles Theorem, these angles are congruent. Also, notice that AC and AD are both radii of the circle which means these segments are congruent. Therefore, according to the Isosceles Triangle Theorem, the base angles of △ ACD are congruent.

Now we can determine the measure of ∠ ADC.

Tangent Line to a Circle

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