1. By Mark Hatem and Maddie Hines
Now what you’ve all been waiting for..
Chapter 12:Circles
By Mark Hatem and Maddie Hines

2. The radius of a circle is a line segment that connects the center of the circle to any point on the same circle.
All radii are equal
A chord is a line segment that connects two points of the circle.
A diameter is a chord that contains the center of the circle.
Lesson 1

4. If a line through the center of a circle is perpendicular to a chord it also bisects the chord.
If a line through the center of a circle bisects a Chord that is not the diameter, it is perpendicular to the chord
Chords in Lesson 1

5. The perpendicular bisector of a chord contains the center of a circle.
One more Chord….

6. A tangent to a circle is a line in the plane of the circle that intersects the circle in exactly one point.
Theorem 59: If a line is tangent to a circle, it is perpendicular to the radius drawn to the point of contact.
Theorem 60: If a line is perpendicular to a radius at its outer endpoint, it is tangent to the circle.
Section 2

7. Lesson 3 – Angles and Arcs
A central angle is an angle whose vertex is the center of the circle.
A reflex angle is an angle whose measure is more than 180°.
The degree measure of an arc is the measure of its central angle.
The arc postulate: If C is on arc AB then mAC + mCB = mAB
Lesson 3 – Angles and Arcs

8. Chords in Lesson 3 In a circle equal chords have equal arcs.
In a circle equal arcs have equal chords.

9. An inscribed angle is an angle whose vertex is on a circle, with each of the angle’s sides intersecting the circle in another point
Theorem 63: An inscribed angle is equal in measure to half its intercepted arc.
Corollary 1 to Theorem 63: Inscribed angles that intercept the same arc are equal.
Corollary 2 to Theorem 63: An angle inscribed in a semicircle is a right angle.
Section 4

10. Lesson 5 - Secants
A secant is a line that intersects a circle in two points.
A secant angle is an angle whose sides are contained in two secants of a circle so that each side intersects the circle in at least one point other than the angle’s vertex.

11. Calculating Secant Angles
A secant angle whose vertex is inside a circle is equal in measure to half the sum of the arcs intercepted by it and its vertical angle.
A secant angle whose vertex is outside a circle is equal in measure to half the difference of its larger and smaller intercepted arc.
150°
89°
(120+89)/2=104.5
(150-10)/2=70
120°
10°

12. If a line is tangent to a circle, then any segment of the line having the point of tangency as one of its end points is a tangent segment to the circle.
Theorem 66, The Tangent Segments Theorem: The tangent segments to a circle from an external point are equal.
Theorem 67, The Intersecting Chords Theorem: If two chords intersect in a circle, 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.
Section 6

13. Intersecting Secants Theorem: If two secants intersect outside of a circle, for each secant, the product of the segment outside the circle and the entire secant are equal.
A(a + b) = c(c + d)
The Intersecting Secant-Tangent Theorem: If a tangent and a secant intersect outside of a circle, then the square of the lengths of the tangent is equal to, for the secant, the product of the segment outside the circle and the entire secant.
T2= a(a + b)
Additional lessons

14. Angles and Arcs in Circles Notes
Central Angle
x°=a°
Angle inscribed in a semicircle
x°=90°
Inscribed angle
x°=1/2a°
Angles inscribed in the same arc
x°=y°=1/2a°
Tangent and radius angle
Tangent and chord angle a x x a a x x y a x x

15. Angles and Arcs in Circles Notes Cnt.
Two Chords
Angle
x°=a°+b° 2
Two tangents
x°=(b-a)/2
Or x°=180-a
Or x°=180-b
Two Secants
x°= b-a
Secant and Tangent a b x x a b b b x a a X

16. #2 Section 2: Know the definition of tangent and know that it creates a right angle with the radius as this will be helpful in problem solving
#1 Section 4: Inscribed angles are one of the most important concepts in the chapter and is found in many problem solving questions and the corollaries are helpful shortcuts
#3 Section 6: Two helpful simple formulas were given
#4 Additional Lesson: Two more formulas are given with regards to secants and tangents
What's Important