Circles

A circle is the set of all points in a plane that are equidistant from a given point called the center of the circle.

circle A, or A

A

A

In a plane, the interior of a circle consists of the points that are inside the circle. The exterior of a circle consists of the points that are outside the circle.

interior

exterior

Segments

and Lines

The segment joining the center to a point on the circle is the radius of the circle.

Example:

Two circles are congruent if they have the same radius measures.

AG or AH

G

A

H

A chord is a segment whose endpoints are points on the circle.

Ex: CD or EF

A

F

E

CD

A diameter is a chord that passes through the center of the circle.

Ex: BC

A

C

B

The distance across the circle, through its center, is the measure of the diameter of the circle. Ex: BC

A

C

B

The length of a diameter is twice the length of a radius. The length of a radius is half the length of a diameter.

A

C

B

A secant is a line that intersects a circle in two points

A tangent is a line in the plane of a circle that intersects the circle in exactly one point. The point at which a tangent line intersects the circle to which it is tangent is the point of tangency.

In a plane, two circles can intersect in two, one or no points. Coplanar circles that intersect in one point are called tangent circles.

Coplanar circles that have a common center are called concentric.

A

A line or segment that is tangent to two coplanar circles is called a common tangent.

A common internal tangent intersects the segment that joins the centers of the two circles.

A common external tangent does not intersect the segment that joins the centers of the two circles.

TheoremsIf a line is tangent to a circle, then it is

perpendicular to the radius drawn to the point of tangency.

In a plane, if a line is perpendicular to a radius of a circle at its endpoint on the circle, then the line is tangent to the circle.

Theorem

If two segments from the same exterior point are tangent to a circle, then they are congruent.

Angles

and Arcs

In a plane, an angle whose vertex is the center of a circle is a central angle of the circle.

A

C

Bcentral angle

If the measure of a central angle is less than 180˚, then B and C and the points of A in the interior of BAC form a minor arc.

minor arc

A

C

B

A

C

B

The measure of a minor arc is defined to be the measure of its central angle.

x x

If the two endpoints of an arc are the endpoints of a diameter, then the arc is a semicircle.

A

C

BD semicircle

The measure of a semicircle is 180˚.

A

C

BD semicircle

A

C

B

If the measure of a central angle is greater than 180˚, then B and C and the points of A in the exterior of BAC form a major arc of the circle.

major arc

The measure of a major arc is defined as the difference between 360˚ and the measure of its associated minor arc.

A

C

B

x(360 – x)

Arc Addition Postulate

The measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs.

A

C

B

D

mBD mDC mBDC

Inscribed

Angles

An inscribed angle is an angle whose vertex is on a circle and whose sides contain chords of the circle. The arc that lies in the interior of an inscribed angle and has endpoints on the angle is called the intercepted arc of the angle.

Measure of an Inscribed Angle

If an angle is inscribed in a circle, then its measure is half the measure of its intercepted arc.

Theorem

If two inscribed angles of a circle intercept the same arc, then the angles are congruent.

Properties of Inscribed Polygons

If all the vertices of a polygon lie on a circle, the polygon is inscribed in the circle and the circle is circumscribed about the polygon.

Theorems About Inscribed Polygons

If a right triangle is inscribed in a circle, then the hypotenuse is a diameter of the circle.

Theorems About Inscribed Polygons

Conversely, if one side of an inscribed triangle is a diameter of the circle, then the triangle is a right triangle and the angle opposite the diameter is the right angle.

A quadrilateral can be inscribed in a circle if and only if its opposite angles are supplementary.

Other Angle Relationships in

Circles

Theorem about Chords of Circles

In the same circle, or in congruent circles, two minor arcs are congruent if and only if their corresponding chords are congruent.

Theorem about Chords of Circles

If a diameter of a circle is perpendicular to a chord, then the diameter bisects the chord and its arc.

Theorem about Chords of Circles

If one chord is a perpendicular bisector of another chord, then the first chord is a diameter.

Theorem about Chords of Circles

In the same circle, or in congruent circles, two chords are congruent if and only if they are equidistant from the center.

Theorem

If a tangent and a secant intersect at a point on a circle, then the measure of each angle is one half the measure of its intercepted arc.

If two chords intersect in the interior of a circle, then the measure of each angle is one half the sum of the measures of the arcs intercepted by the angle and its vertical angle.

F

A

E

B

C

D

12

m CFB mCB mDE

12

m DFE mCB mDE

F

A

E

B

C

D

12

m CFD mCD mBE F

A

E

B

C

D

12

m BFE mCD mBE

If a tangent and a secant intersect in the exterior of a circle, then the measure of the angle formed is one half the difference of the measures of the intercepted arcs.

12

m BVA mBA mCA

If two tangents intersect in the exterior of a circle, then the measure of the angle formed is one half the difference of the measures of the intercepted arcs.

12

m BVA mBCA mBA

If two secants intersect in the exterior of a circle, then the measure of the angle formed is one half the difference of the measures of the intercepted arcs.

12

m BVA mBA mDC

Segment Lengths in Circles

If two chords intersect in the interior of a circle, then the product of the lengths of the segments of one chord is equal to the product of the lengths of the segments of the other chord.

Ex: 8x = 24

x = 3

8 × 3 = 4 × 6

If two secant segments share the same endpoint outside a circle, then the product of the length of one secant segment and the length of its external segment equals the product of the length of the other secant segment and the length of its external segment.

Ex: 4(x + 4) = 3(3 + 5)

4x + 16 = 24

4x = 8

x = 2

4(2 + 4) = 3 (3 + 5)

outside segment × whole segment = outside segment × whole segment

If a secant segment and a tangent segment share an endpoint outside a circle, then the product of the length of the secant segment and the length of its external segment equals the square of the length of the tangent segment.

Ex: 62 = 4(4 + x)

36 = 16 + 4x

20 = 4x

5 = x

62 = 4(4 + 5)

tangent squared = outside segment × whole segment

Equations of Circles

You can write an equation of a circle in a coordinate plane if you know its radius and the coordinate of its center.

Suppose the radius of a circle is r and the center is (h, k). Let (x, y) be

any point on the circle.

The distance between (x, y) and (h, k) is r, so you can use the

Distance Formula.

2 2( ) ( )r x h y k

Square both sides to find the standard equation of a circle with radius r and center (h, k).

(x – h)2 + (y – k)2 = r 2

If the center is the origin, then the standard equation is x 2 + y 2 = r 2