Lesson 4 of 15
The three angles come to 180°
Show on a big screenCut a small paper triangle. Then cut a big one of the same shape. The corners match. So where is the extra angle?
Step 1 of 4
- A triangle, three corners.
- The three corners measure 45°, 55° and 80°.
- A straight line appears, with a point O on it.
- Copied onto O, the corners fill the line: 45° + 55° + 80° = 180°.
Three corners, one total
There is none. Both triangles have corners that add to 180°.
The three angles of any triangle add to exactly 180°. This is the angle sum property.
Place the three corners side by side at one point. Together they fill a straight line.
A triangle has at most one right angle. It has at most one obtuse angle. A second one would make the total more than 180°.
Fold the corners onto a line. Check the total.
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Try the lab to finish this lesson.