Lesson 3 of 15
Named by its angles
Show on a big screenHold a set square against each paper triangle. The set square has a square corner of exactly 90°. Some triangle corners are narrower. One may be wider.
The widest corner names it
Look at the widest corner.
| Widest corner | Name | Corners of an example |
|---|---|---|
| Under 90° | Acute-angled | 60°, 60°, 60° |
| Exactly 90° | Right-angled | 90°, 36.9°, 53.1° |
| Over 90° | Obtuse-angled | 134.8°, 22.6°, 22.6° |
An acute-angled triangle has all three corners acute.
The square corner can face any direction. A right-angled triangle need not have a flat bottom.
Open the widest corner. Watch the name change.
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Try the lab to finish this lesson.