Angles
An angle measures the amount of turn between two intersecting lines, expressed in degrees (°). Complementary angles sum to 90°, supplementary angles sum to 180°, and the three angles inside any triangle always total exactly 180°. These fundamental angle relationships form the foundation for solving geometric problems across mathematics.
Why it matters
Angle calculations appear throughout engineering, architecture, and navigation. Architects use complementary angles when designing 90° corners in buildings, whilst surveyors rely on triangle angle sums to measure distances across terrain. In Year 6 SATs, pupils encounter basic angle facts, progressing to GCSE Foundation where algebraic angle problems test understanding of straight-line relationships. Carpenters use 45° complementary angles for perfect mitre joints, and satellite dishes require precise angular positioning to within 2° for optimal signal reception. Computer graphics render 3D objects using angle matrices, whilst GPS systems calculate positions through triangulation using known angle measurements. Understanding these relationships enables problem-solving in subjects from physics (light refraction at 30° angles) to art (perspective drawing using vanishing points).
How to solve angles
Angles
- Complementary angles sum to 90°.
- Supplementary angles sum to 180°.
- Triangle angles sum to 180°.
- Angles on a straight line sum to 180°.
Example: If one angle is 40°, its complement is 50°.
Worked examples
Two angles are complementary. One is 22°. Find the other.
Answer: 68°
- Complementary angles add to 90° → 90° − 22° = 68° — Subtract 22 from 90.
Two angles are complementary. One is 48°. Find the other.
Answer: 42°
- Complementary angles sum to 90° → 90° − 48° = 42° — Subtract from 90.
A triangle has angles 20° and 66°. Find the third angle.
Answer: 94°
- Angles in a triangle sum to 180° → 180° − 20° − 66° = 94° — Subtract known angles from 180.
- Verify → 20° + 66° + 94° = 180° ✓ — Check the sum.
Common mistakes
- Confusing complementary and supplementary pairs leads to errors like finding 130° as the complement of 50° instead of the correct supplement of 130°
- Triangle angle calculations often produce mistakes such as finding 120° for the third angle when given 40° and 80°, instead of the correct 60°
- Straight-line angle problems frequently result in errors like calculating 220° for angles on a line instead of recognising the sum must equal 180°