Complementary and Supplementary
Pairs summing to 90° or 180°.
Key idea
Pairs summing to 90° or 180°.
In depth
Complementary angles sum to 90°, forming a right angle. Supplementary angles sum to 180°, forming a straight line. Knowing one angle gives you the other by subtraction.
Worked example
The complement of 35° is 55°. The supplement of 110° is 70°.
Formula
Real-world uses
- Working out missing angles on a ladder
- The angles around a straight road junction
Common mistake
Mixing up the two words. Complementary → 90, Corner; Supplementary → 180, Straight.
How to learn this
- Read the key idea and say it in your own words.
- Work through the worked example, then try a similar problem.
- Use the interactive lesson’s visual and mini-quiz to check the pattern.
Frequently asked questions
What is Complementary and Supplementary?
Pairs summing to 90° or 180°. It is a topic at the beginner level in Geometry.
What is the formula for Complementary and Supplementary?
The key formula is a + b = 90°, a + b = 180°.
Where is Complementary and Supplementary used in real life?
Real-world examples: Working out missing angles on a ladder; The angles around a straight road junction.
What should I learn before Complementary and Supplementary?
Make sure you are comfortable with Measuring Angles with a Protractor, the previous topic in Geometry.
What is a common mistake with Complementary and Supplementary?
Mixing up the two words. Complementary → 90, Corner; Supplementary → 180, Straight.
How can I practise Complementary and Supplementary?
Open the interactive lesson, play with the visual, then answer the three-question mini-quiz. Getting two right marks the topic complete.
What comes after Complementary and Supplementary?
Move on to Vertical Angles next.
Related beginner topics in Geometry
- Points, Lines and Planes – The undefined building blocks of geometry.
- Line Segments and Rays – A segment has two endpoints; a ray starts at a point and goes on forever.
- Angles – Two rays sharing an endpoint; measured in degrees.
- Types of Angles – Acute <90°, right 90°, obtuse between 90° and 180°, straight 180°.
- Measuring Angles with a Protractor – Align the baseline and read the scale.
- Triangles: Classification – By sides (equilateral, isosceles, scalene) and by angles.