Idea 18of 24Builds onInside and outside angles
Outside angles make a full turn
What do a triangle's three outside angles add up to?
Today you'll discover
- what happens when you walk round a triangle
- the total of the three outside angles
- how to find a missing one
Think firstWalk round A, B, C and back
Walk round the triangle, turning at each corner. How far have you turned when you are back?
Decide first, then press Next. Watch the three outside slices close a circle.
A triangle with inside angles 70°, 50° and 60°, its side carried on past B. Then every side is carried on, giving outside angles of 130° at B, 120° at C and 110° at A, and the three slices slide over and fit round one point, closing a whole circle: 360°. Then a wide triangle does the same. The last step shows outside angles of 100° and 150° and a question mark for the third.
See the whole ideathe big idea and the key word, in one place
The big idea
The three exterior angles of a triangle always add up to 360°, a full turn.
Walk itWalk round a triangle drawn on the playground, turning at each corner. Back at the start, facing the same way, you have turned 360°.
Compare themInside angles: 70° + 50° + 60° = 180°. Outside angles: 110° + 130° + 120° = 360°.
Key word
- full turn
- turning all the way round to face the same way again: 360°
Ready to use it?
- Find a missing exterior angle from the other two.