What if your family went to Washington DC over the summer and saw the White House? The closest you can get to the White House are the walking trails on the far right. You got as close as you could (on the trail) to the fence to take a picture (you were not allowed to walk on the grass). Where else could you have taken your picture from to get the same frame of the White House? Where do you think the best place to stand would be?
Your line of sight in the camera is marked in the picture as the grey lines. The white dotted arcs do not actually exist, but were added to help with this problem.
After completing this Concept, you will be able to use inscribed angles to answer this question.
CK-12 Foundation: Chapter9InscribedAnglesinCirclesA
Learn more about inscribed angles by watching the video at this link.
is an angle with its vertex is the circle and its sides contain chords. The
is the arc that is on the interior of the inscribed angle and whose endpoints are on the angle. The vertex of an inscribed angle can be anywhere on the circle as long as its sides intersect the circle to form an intercepted arc.
Let's investigate the relationship between the inscribed angle, the central angle and the arc they intercept.
Investigation: Measuring an Inscribed Angle
Tools Needed: pencil, paper, compass, ruler, protractor
1. Draw three circles with three different inscribed angles. For
, make one side of the inscribed angle a diameter, for
inside the angle and for
outside the angle. Try to make all the angles different sizes.
2. Using your ruler, draw in the corresponding central angle for each angle and label each set of endpoints.
3. Using your protractor measure the six angles and determine if there is a relationship between the central angle, the inscribed angle, and the intercepted arc.
Inscribed Angle Theorem:
The measure of an inscribed angle is half the measure of its intercepted arc.
In the picture,
. If we had drawn in the central angle
, we could also say that
because the measure of the central angle is equal to the measure of the intercepted arc. To prove the Inscribed Angle Theorem, you would need to split it up into three cases, like the three different angles drawn from the Investigation.
Congruent Inscribed Angle Theorem:
Inscribed angles that intercept the same arc are congruent.
Inscribed Angle Semicircle Theorem:
An angle that intercepts a semicircle is a right angle.
In the Inscribed Angle Semicircle Theorem we could also say that the angle is inscribed in a semicircle. Anytime a right angle is inscribed in a circle, the endpoints of the angle are the endpoints of a diameter. Therefore, the converse of the Inscribed Angle Semicircle Theorem is also true.
From the Inscribed Angle Theorem,
The intercepted arc for both angles is
is the center,
is a diameter. Therefore,
inscribes semicircle, or
Watch this video for help with the Examples above.
CK-12 Foundation: Chapter9InscribedAnglesinCirclesB
Concept Problem Revisited
You can take the picture from anywhere on the semicircular walking path. The best place to take the picture is subjective, but most would think the pale green frame, straight-on, would be the best view.
is the set of all points that are the same distance away from a specific point, called the
is the distance from the center to the circle. A
is a line segment whose endpoints are on a circle. A
is a chord that passes through the center of the circle. The length of a diameter is two times the length of a radius. A
is an angle formed by two radii and whose vertex is at the center of the circle. An
is an angle with its vertex on the circle and whose sides are chords. The
is the arc that is inside the inscribed angle and whose endpoints are on the angle.
by the Congruent Inscribed Angle Theorem.
from the Inscribed Angle Theorem.
by the Inscribed Angle Semicircle Theorem.
from the Inscribed Angle Theorem.
, we need to find
is the third angle in
, which means that
Fill in the blanks.
An angle inscribed in a ________________ is
Two inscribed angles that intercept the same arc are _______________.
The sides of an inscribed angle are ___________________.
Draw inscribed angle
. Then draw central angle
. How do the two angles relate?
Find the value of
is a diameter of a circle centered at
is any other point on the circle. Draw the line through
that is parallel to
, and let
be the point where it meets
. Explain why
is the midpoint of
Fill in the blanks of the Inscribed Angle Theorem proof.