Mathematics Class 9
Lines and Angles
Prove the angle facts you once measured, and when a converse needs its own proof.
11 lessons · ~88 min total
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Lessons in this topic
- 0 Start here Linear pairs, crossing lines, parallel lines and a transversal, and which fact to use. · 4 min You are here
- 1 The first theorem Two angles side by side on a straight line total 180°. They are a linear pair. · 8 min
- 2 The other direction Two angles that total 180° need not sit on one line. A converse is a new claim. · 8 min
- 3 Two pairs at every crossing Where two lines cross, the angles facing each other are equal. · 8 min
- 4 One agreement, three results On parallel lines, corresponding angles are equal. Alternate and co-interior angles follow. · 9 min
- 5 The converse has to earn its own argument If corresponding or alternate angles are equal, the lines are parallel. · 10 min
- 6 Parallelism passes along Two lines that are each parallel to a third line are parallel to each other. · 9 min
- 7 Reasons, not looks Start from parallel lines to find angles. Start from equal angles to show parallel lines. · 9 min
- 8 What each fact hides Each angle fact is always true, true on parallel lines only, or false as stated. · 9 min
- 9 The big picture Eight lessons, two accepted facts: a chain of proofs and two contradictions that ran the facts backwards. · 6 min
- 10 Track quiz Cheat-sheet table plus a reveal quiz — eight questions from across the track. · 8 min
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