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{{ printedBook.courseTrack.name }} {{ printedBook.name }} # Circle Theorems

An angle can be constructed by the lines and line segments that intersect a circle. In this lesson, the angles related to circles and their properties will be explored.

### Catch-Up and Review

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

## Investigating Inscribed Angles of a Circle

Consider a circle with a radius of units. An angle whose sides are two chords of the circle is formed as shown. Move the points and along the circle so that the angle is a right angle. • When the measure of the angle is what can the chord joining and be called?
• Does the arc that the angle intercepts have a special name? If yes, what is it?

## Inscribed and Central Angles of a Circle

On the circle above, construct an angle with a vertex at the center of the circle. The angle being constructed should also cut off the same arc. In other words, construct the corresponding central angle. Recall that the measure of an arc is the measure of the central angle that includes the arc. Now, move the points and make conjectures that explain the relationship between the measures of angles and arcs.

## Inscribed Angle of a Circle

In the circles shown previously, the angle formed by two chords is called an inscribed angle.

An inscribed angle is an angle whose vertex lies on the circumference of a circle and whose sides are secant to the circle. ## Inscribed Angle Theorem

Observe the measures of the angle and the arc intercepted by the angle. Start by moving so that and are collinear. Then, move it once again so that and are collinear. As can be seen, when and are collinear, becomes the diameter of the circle, and the angle cuts off the semicircle. Furthermore, the measure of is twice the measure of an inscribed angle that intercepts it. This statement can be restated as a theorem.

The measure of an inscribed angle is half the measure of its intercepted arc. In this figure, the measure of is half the measure of

### Proof

An inscribed angle can be created in three different ways depending on where the angle is located in relation to the center of the circle. Consider Case II. A diameter can be drawn from through dividing the inscribed angle into two. Let the measures of and be and respectively. Because the radii of a circle are all congruent, two isosceles triangles can be obtained by drawing and Therefore, by the Isosceles Triangle Theorem, the measures of and will also be and respectively. By the Triangle Exterior Angle Theorem, it is recognized that and The measure of an intercepted arc is the same as the measure of its central angle. Therefore, the measures of and are and respectively. From here, using the Angle Addition Postulate and the Arc Addition Postulate, and can be written in terms of and Consequentially, the measure of is half the measure of The proof of Case II has been completed.

By applying similar logic as the procedure above, Case I and Case III can be proven.

## Finding the Inscribed Angle of a Circle

In the diagram, the vertex of is on the circle and the sides of the angle are chords of the circle. Given the measure of , find the measure of the angle. Write the answer without the degree symbol.

### Hint

The Inscribed Angle Theorem can be used to find the measure of the angle.

### Solution

The angle shown in the diagram fits the definition of an inscribed angle. For this reason, the measure of the angle can be found using the Inscribed Angle Theorem. The theorem states that the measure of an inscribed angle is half the measure of its intercepted arc. The arc intercepted by measures
Therefore, measures

## Practice Finding Inscribed Angles

Find the measure of the inscribed angle in the circle. ## Finding the Central Angle of a Circle

Similarly, given the measure of an inscribed angle, the measure of its corresponding central angle can be found using the Inscribed Angle Theorem. This can be done because the measure of the central angle is the same as the measure of the arc that the central angle cuts off.

In the circle, measures Find the measure of the corresponding central angle.

### Hint

Start by drawing the corresponding central angle.

### Solution

Recall that a central angle is an angle whose vertex lies at the center of the circle. Additionally, the inscribed angle and its corresponding central angle intercept the same arc for this example. Therefore, the corresponding central angle is The arc is included in Therefore, the measure of is equal to the measure of Using the Inscribed Angle Theorem, the measure of can be found.
Solve for
Therefore, also measures

## Practice Finding Central Angles

Given the measure of an inscribed angle, find the measure of its corresponding central angle. ## Inscribed Angles of a Circle Theorem

Up to now, the relationship between inscribed angles and their corresponding central angles has been discussed. Now the relationship between two inscribed angles that intercept the same arc will be investigated. As can be observed, the angles are congruent, so long as they intercept the same arc.

If two inscribed angles of a circle intercept the same arc, then they are congruent. By this theorem, in the above diagram and are congruent angles.

### Proof

Consider two inscribed angles that intercept the same arc in a circle. By the Inscribed Angle Theorem, the measures of and are half the measure of

Therefore, by the Transitive Property of Equality, it can be said that these two angles have the same measure. Consequently, by the definition of congruent angles, and are congruent.

## Using the Inscribed Angles of a Circle Theorem

Mark and Jordan have been asked to find the measure of Mark claims that the measure of is the same as the measure of Jordan, however, thinks that its measure is half the measure of Who is correct?
Find the measure of

### Hint

Determine which angles intercept the same arc. Use the Inscribed Angles of a Circle Theorem to find .

### Solution

The inscribed angles and intercept By the Inscribed Angles of a Circle Theorem, is congruent to Accordingly, half the measure of can be found as Additionally, and intercept the same arc, Again by the Inscribed Angles of a Circle Theorem, is congruent to Therefore, Since neither Mark's claim nor Jordan's claim is correct.

## Constructing Angles Outside of a Circle

Inscribed angles, or the central angles, are not the only angles related to circles. In the next part, the angles constructed outside the circles will be examined. To construct an angle outside a circle, tangents can be used.

## Tangent

A tangent is a line, segment, or ray that intersects a circle at exactly one point. The point is called point of tangency, and the line, segment, or ray is said to be tangent to the circle. ## Tangent to Circle Theorem

A line is tangent to a circle if and only if the line is perpendicular to a radius of the circle at its endpoint on the circle. In the diagram, is tangent to the circle

Line is tangent to

### Proof

The theorem will be proven in two parts as it is a biconditional statement. Each part will be proven by using an indirect proof.

### Part

Assume that line is tangent to the circle and is not perpendicular to By the Perpendicular Postulate, there is another segment from that is perpendicular to Let this segment be The aim is to show that must be this segment. Since is perpendicular to then is a right triangle. In this right triangle, whose length is units, is the hypotenuse and therefore the longest side. Consequently, is less than In the diagram, it can be seen that contains a radius of Because part of the segment lies outside the circle, the unknown portion can be assigned another variable, By the Segment Addition Postulate, is equal to the sum of and These two expressions, together with can be substituted into the above inequality.
Since a length cannot be less than the assumption that is not perpendicular to the line must be false. Therefore, is perpendicular to the tangent at its endpoint on the circle. This concludes the first part of the proof.

Line is tangent to

### Part

For the second part, it will be assumed that is perpendicular to the radius at and that line is not tangent to In this case, line intersects at a second point Since is perpendicular to at is a right triangle and is the hypotenuse. Therefore, must be less than However, both and are radii of This means that they must have the same length. These two statements contradict each other. Therefore, both cannot be true. The contradiction came from supposing that line was not tangent to Consequently, is a tangent line to the circle. This completes the second part of the proof.

line is tangent to

Having proven both parts completes the proof for the theorem.

Line is tangent to

The method used to prove the theorem is called indirect proof.

## Using the Tangent to Circle Theorem

In the diagram, is tangent to the circle at the point and is a diameter. If the measure of is Find the measure of

### Hint

A tangent is perpendicular to the radius of a circle through the point of tangency.

### Solution

It has been given that By the Tangent to Circle Theorem, is perpendicular to In other words, Since the sum of the measures of the interior angles of a triangle is Notice that is an inscribed angle on the diameter of Therefore, measures Once again, using the Triangle Sum Theorem, the measure of can be found to be Since is associated with and the measure of the arc can be found using the Inscribed Angle Theorem.
Solve for

## Circumscribed Angle Theorem

When two tangents of a circle intersect at an exterior point, the angle formed is called a circumscribed angle. In the diagram above, and are tangents to the circle and is a circumscribed angle.

A circumscribed angle is supplementary to the central angle it cuts off.

The measure of a circumscribed angle is equal to minus the measure of the central angle that intercepts the same arc. Considering the above diagram, the following relation holds true.

### Proof

By definition, a circumscribed angle is an angle whose sides are tangents to a circle. Since is a circumscribed angle, and are tangents to at points and respectively. By the Tangent to Circle Theorem, is perpendicular to and is perpendicular to Notice that is a quadrilateral and two of its angles are right angles. Recall that the sum of all of the angles in a quadrilateral is Substituting the known angle measures and solving for will give the desired equation.
Solve for
This completes the proof.

## Practice Using the Circumscribed Angle Theorem

Find the measure of the central angle. ## Using Circle Theorems to Solve Problems

The following example involving circumscribed angles and inscribed angles could require the use of the previously learned theorems.

Two tangents from to are drawn. The measure of is Find the measure of the inscribed angle that intercepts the same arc as

### Solution

The inscribed angle that intercepts is By the Inscribed Angle Theorem, is half Recall that the measure of an arc is the same as the measure of its corresponding central angle. Therefore, is equal to Substitute into the above equation. Now, by the Circumscribed Angle Theorem, is minus It is given that Substituting that measure into the equation will give the measure of the inscribed angle.
Evaluate right-hand side

## Summarizing Angle Theorems Related to Circles

This lesson defined three angles related to circles as well as the relationships between these angles. The diagram below shows the definitions and the main theorems of this lesson. 