Angles on parallel lines
Alternate and corresponding angles are equal; co-interior angles add to 180°.
The common mistake
Calling co-interior (same-side) angles equal when they actually add to 180°.
Instead: Z-shape: equal (alternate). F-shape: equal (corresponding). C/U-shape: sum to 180° (co-interior).
Worked example with Guide me
A line crosses two parallel lines. One co-interior angle is 115°. Find the other.
- Sugar · Co-interior angles — equal or add to 180°?Student · They add to 180°.
- Sugar · So?Student · 180 − 115 = 65°.
Answer: 65°
Try 3 practice questions
1. Alternate angle to 72°?
Show answer
72°
2. Co-interior partner of 100°?
Show answer
80°
3. Corresponding angle to 130°?
Show answer
130°
Quick answers
- What is the most common mistake with angles on parallel lines?
- Calling co-interior (same-side) angles equal when they actually add to 180°.
- How do I get angles on parallel lines right?
- Z-shape: equal (alternate). F-shape: equal (corresponding). C/U-shape: sum to 180° (co-interior).
- Can you show a worked example?
- A line crosses two parallel lines. One co-interior angle is 115°. Find the other. 65°
Related concepts
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