Complementary and Supplementary Angles


Complementary and supplementary angles are a key part of any geometry curriculum. When I taught high school, I was always surprised that my students would get these terms confused. They should know these angles from their middle school math class.

However, I started to think about all the new terms they learn in geometry. There are corresponding angles, consecutive angles, adjacent angles, alternate exterior angles, etc.

With all these new terms, it might be difficult for students to remember even the most basic geometry terms.


Trick for Remembering Complementary and Supplementary Angles

To help students remember the difference between complementary and supplementary angles, I taught them this quick trick.

For complementary, students can change the “C” to a “9” to help them remember 90°.


For supplementary, students can change the “S” to an “8” to help them remember 180°.


This “trick” really helped my students! I would see them write out C and S on their test and change the letters to 9 and 8!

Another Helpful Trick

Another way to help students remember the difference between complementary and supplementary angles is this:

The “C” in complementary stands for “Corner” like a right angle.

The “S” in supplementary stands for “Straight” like a line.

Resources for Teaching Angles

Here are some resources you may like for teaching angles:

I hope this post was helpful!

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Comment below and tell me if your students confuse complementary and supplementary angles!



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