Angles & Parallel Lines
Know which angles are equal and which add to 180°.
The rules
- Vertical angles are equal. Angles on a straight line add to 180°.
- A transversal crossing parallel lines makes equal corresponding and alternate interior angles.
- Same-side interior angles add to 180°.
- Triangle angles add to 180°.
Watch out: Assuming lines are parallel when the problem doesn't say so.
Worked example
Two parallel lines are cut by a transversal. One angle is 65°. What is its same-side interior partner?
- Same-side interior angles add to 180°.
- 180 − 65 = 115.
Answer: 115°
Try it yourself
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