1. Space and Shape
Grade 9 Math

2. Discovering Circle Theorems
Recognize, name, describe and represent: arcs, chords, tangents, central angles, inscribed angles and circumscribed angles. Make generalizations about their relationships in circles.

3. To review for this lesson…
Circumference: the distance outside of a circle
Diameter: the distance across the longest part of a circle
Radius: the distance from the center to the outside of a circle
Segment: essentially a “chunk” of circle, made of 2 radii and an arc
Centre: the most central point in a circle, equally distant from all other parts
Chord: a line that crosses the circle, but does not divide into equal parts (not the diameter)

4. Parts of the Circle Circumference Diameter Chord Radius Sector Center
Tangent Arc

5. Activity #1 c d a b Mark two points a and b on the circumference.
Draw lines from a and b to another point on the circumference. Call this ‘c’
Draw lines from a and b to the centre. Call this ‘d’. 
4. Measure angle acb. Measure angle adb d x a b
What can you observe about the angle at the centre and angle at the circumference?

6. Activity #2
1. Draw the diameter of the circle to form 2 semi-circles
2. In one half, mark any point (x) on the circumference and draw lines from each end of the diameter to that point
3. Measure the angle you have created.
4. Repeat the process in the second half of your circle x x x
Now we have two triangles. What can you say about the triangle formed in a semi-circle?

7. Activity #3 b
1. Mark three points a, b and c anywhere in the top half of the circumference and then points e and f in the bottom half, again on the circumference
2. Draw 2 straight lines each from a, b, and c to points e and f. (six lines in total)
3. Measure the three angles eaf, ebf and ecf. c a e f
What can you say about the relationship between the three angles?

8. Activity #4
Draw a quadrilateral where all four vertices are on the circumference of the circle.
Measure the angle of each vertex in your quadrilateral. x x x x
What do you notice about opposite angles in your cyclic quadrilateral?

9. Activity #5 x Draw the radius on your circle.
Draw a tangent which touches the circumference where the radius does.
Measure the angle between the radius and the tangent.
Repeat the process again at a different point on the circumference x
What do you notice about the angle created?

10. Activity #6
Draw two tangents to the circle which cross at a point outside of the circle.
Measure the distance from the where each tangent meets the circle to the point.
What do you notice about the lengths?

11. To clarify….
v=WufDdk1MZTg

12. Watch for this one later.
_____________: an angle made by…
The angle subtended at the centre of a circle (by an arc or chord) _____________by the same arc or chord. (angle at centre)
Theorem 1 xo o
2xo
Watch for this one later.

13. Th2 Theorem 2 The angle in a _____________is a right angle
Or, the angle _____________to the circumference is a right angle
Theorem 2 o
Diameter
This is just a special case of Theorem 1 and is referred to as a theorem for convenience.
Th2

14. Angles subtended by an arc or chord in the same _____________
Theorem 3 yo xo xo
Th3

15. Cyclic Quadrilateral Theorem
The _____________of a cyclic quadrilateral are supplementary. (They sum to 180o) w x
Th6 q p y z r s

16. The angle between a tangent and a _____________ (Tan/rad)
Theorem 5 o

17. Two Tangent Theorem
Theorem 6
From any point outside a circle _____________ can be drawn and they are equal in length. Q R P P T U Q R
PT = PQ T U
PT = PQ
Th7

18. The Alternate Segment Theorem
The angle between a _____________ through the point of contact is equal to the angle subtended by that chord in the alternate segment.
45o xo yo
60o zo
**Find the missing angles below giving reasons in each case. xo yo yo xo
Th5