Types of angle
- Acute: less than 90°. Right angle: exactly 90°. Obtuse: between 90° and 180°. Reflex: between 180° and 360°.
- Measure and draw angles with a protractor, starting from 0 on the correct scale.
Angle facts
- Angles on a straight line add up to 180°. Angles around a point add up to 360°.
- Vertically opposite angles are equal.
- Angles in a triangle add up to 180°. Angles in a quadrilateral add up to 360°.
- An isosceles triangle has two equal angles; an equilateral triangle has three angles of 60°.
Parallel lines
- Alternate angles are equal (a Z shape).
- Corresponding angles are equal (an F shape).
- Co-interior angles add up to 180° (a C shape).
Polygons and bearings
- The interior angles of a polygon with n sides add up to (n − 2) × 180°: a pentagon's add up to 540°.
- The exterior angles of any polygon add up to 360°. In a regular polygon, each exterior angle is 360° ÷ n, and interior + exterior = 180°.
- Bearings are measured clockwise from north and written with three figures: east is 090°.
Key terms
- Acute angle
- An angle less than 90°.
- Right angle
- An angle of exactly 90°.
- Obtuse angle
- An angle between 90° and 180°.
- Reflex angle
- An angle between 180° and 360°.
- Vertically opposite angles
- The equal angles opposite each other where two lines cross.
- Alternate angles
- Equal angles on opposite sides of a line crossing parallel lines (Z shape).
- Corresponding angles
- Equal angles in matching positions on parallel lines (F shape).
- Co-interior angles
- Angles between parallel lines on the same side, which add up to 180°.
- Interior angle
- An angle inside a polygon.
- Exterior angle
- The angle between one side of a polygon and the next side extended.
- Bearing
- A direction measured clockwise from north, with three figures.