Maths · Geometry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Most angle questions are solved with a small set of facts. Angles at a point add up to 360°, angles on a straight line add up to 180°, the angles of a triangle add up to 180° and the angles of a quadrilateral add up to 360°.
When a line crosses two parallel lines, it makes equal angles and angles that add up to 180°. Learn the three names: alternate, corresponding and co-interior.
For any polygon, the exterior angles add up to 360°. This is the quickest way into regular polygon questions: find one exterior angle first, then the interior angle next to it, because the two lie on a straight line.
Students set co-interior angles equal to each other. Only alternate and corresponding angles are equal; co-interior angles add up to 180°.
Answer: 18 sides
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