circle theorems 1 10c3 01.02.12
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SFR Lesson NotesThe Winston Churchill School
Circle Theorems
Geometry
38o
x
76o
x
1
2
3
70o
x
65o
x
4
Starter
Draw a circle. Draw a diameter and label it AB.
Mark any point C on the circumference. Join A, B and C to form a triangle.
Measure angle ACB. What do you find?
Repeat these steps to establish a rule.
= Circle Theorem 1: Angles in a semicircle are 90o
Angles in a semicircle:
Draw a circle. Mark any four points on the
circumference and label them A, B, C and D. Join A, B, C and D to form a
quadrilateral. Measure the angles inside the
quadrilateral. Add the opposite angles together.
What do you find? Repeat these steps to establish a rule.
= Circle Theorem 2: Opposite angles in a cyclic quadrilateral + to 180o
Cyclic Quadrilateral
Need help?
mymaths: Circle Theorems
online textbook: pages 138 141
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