When a straight line crosses two parallel lines, it makes eight angles in two matching groups. Corresponding angles are in the same position at each crossing and are equal. Alternate angles are on opposite sides of the crossing line, between the parallel lines, and are equal.
Co-interior angles, also called same-side interior angles, are between the parallel lines on the same side, and they add up to 180°. Use these facts with angles on a line and in a triangle to find missing angles, and give a reason for each step.
Worked example
Two parallel lines are crossed by a line. One angle is 65°. Find the co-interior angle.
- Co-interior angles add to 180°.
- 180 − 65 = 115.
Answer:115°
Key words
- corresponding angles
- equal angles in matching positions (F shape)
- alternate angles
- equal angles in a Z shape
- co-interior angles
- add to 180° (C shape)
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Common Core-referenced: 8.G.A.5