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§ Geometry

Angles

§ Geometry

Angles

CCSS.4.MDCCSS.7.GCCSS.8.G3 min read

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.

§ 01

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).

§ 02

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°.

§ 03

Worked examples

Beginner§ 01

Two angles are complementary. One is 22°. Find the other.

Answer: 68°

  1. Complementary angles add to 90° 90° − 22° = 68° Subtract 22 from 90.
Easy§ 02

Two angles are complementary. One is 48°. Find the other.

Answer: 42°

  1. Complementary angles sum to 90° 90° − 48° = 42° Subtract from 90.
Medium§ 03

A triangle has angles 20° and 66°. Find the third angle.

Answer: 94°

  1. Angles in a triangle sum to 180° 180° − 20° − 66° = 94° Subtract known angles from 180.
  2. Verify 20° + 66° + 94° = 180° ✓ Check the sum.
§ 04

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°
§ 05

Frequently asked questions

What is the difference between complementary and supplementary angles?
Complementary angles add up to 90° (like 30° and 60°), whilst supplementary angles add up to 180° (like 70° and 110°). Complementary angles form a right angle when placed together, supplementary angles form a straight line.
How do you find a missing angle in a triangle?
Subtract the two known angles from 180°. For example, if a triangle has angles of 45° and 65°, the third angle equals 180° − 45° − 65° = 70°. Always check that all three angles sum to exactly 180°.
Why do angles on a straight line equal 180°?
A straight line represents half a complete rotation (360° ÷ 2 = 180°). Any angles formed above the line must combine with those below to recreate the full straight line, totalling 180°.
Can complementary angles be larger than 90°?
No, complementary angles must each be less than 90° since they add to 90°. If one angle equals 90°, its complement would be 0°. Angles larger than 90° can only be part of supplementary pairs.
How do you check angle calculations are correct?
Add all calculated angles to verify they meet the required sum: 90° for complementary pairs, 180° for supplementary pairs or triangles, or 180° for straight-line arrangements. The total should exactly match the expected value.
§ 06

See also

§ 06

Where to next?

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