- Definition
Supplementary angles are two angles that add up to exactly 180 degrees, forming a straight line if placed side-by-side.
If one angle is 120°, its supplementary angle is 60°, because the two angles add up to 180°.
How to teach this in 30 seconds
Try this analogy: Supplementary angles are like opening a book and laying it perfectly flat — the two angles on either side of the spine add up to a straight line.
Ask your child: If we draw a straight line, then another line coming out of it, what about the two angles?
Common mix-up: Kids often confuse supplementary (180°) with complementary (90°). Remember that supplementary angles combine to form a straight line.
Examples
- An angle of 45° and an angle of 135° are supplementary.
- The two adjacent angles on a straight line are always supplementary.
supplementary angles is a term used in Ontario’s Kindergarten to Grade 8 curriculum.
Where this shows up
The descriptions on this page are SmartStudy’s own original, plain-language writing — not the Ministry’s wording. They are aligned to the Ontario Curriculum, © King’s Printer for Ontario. For the Ministry’s official wording, see the source link below. SmartStudy is an independent service and is not affiliated with, endorsed by, or sponsored by the Government of Ontario or the Ministry of Education. Ontario Curriculum and Resources ↗
