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A ray has a start point and no end point. A ray starts at a point on a straight line. It makes two angles. Do they always total 180°?
This topic is Lines and Angles. You measured these facts before. Now you will explain why they are true.
What you will learn
- Linear pair. Two angles on a straight line total 180°.
- The other way round. Angles can total 180° without a line.
- Crossing lines. The angles facing each other are equal.
- Parallel lines. A transversal is a line across them. It makes corresponding, alternate and co-interior angles.
- Equal angles. Equal corresponding or alternate angles show two lines are parallel.
- Same line. Lines parallel to one line are parallel to each other.
- Which fact. Start from parallel lines, or start from equal angles.
- When is it true. Some facts hold always. Some hold on parallel lines. Some are false.
Need a quick recap?
These earlier ideas help. You may skip them if you know them.
- Angles on a line. Angles on a straight line add to 180°.
- A line across two lines. It is called a transversal.
- Parallel lines. Lines on a flat page that never meet.