Angles in a triangle add up to 180 degrees - exercises and proof

We look at several problems with unknown angles that are solved by using the principle that the angle sum in a triangle is 180 degrees. While the problems can be solved in one's head, I also write an equation for each, since at this (pre-algebra) level students need to practice writing and solving simple equations.

Then, we have a drawing problem: draw an isosceles triangle with 20-cm legs and 40-degree base angles.

Lastly I show you a simple and elegant proof for the fact that the angle sum in a triangle is 180 degrees (or a straight angle). In the proof we use a line that is parallel to the base of our generic triangle. There will be a pair of vertical angles and some corresponding angles formed... check the video to see it all!

See also

Unknown angle problems — video lesson

Math Mammoth Geometry 3 — a self-teaching worktext with explanations & exercises (grades 4-5)

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