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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.