Idea 17of 24Builds onFind the missing angleAcute, right, obtuse or straight
Inside and outside angles
What is the angle just outside a triangle's corner?
Today you'll discover
- how to make an outside angle
- how inside and outside angles fit together
- how to find one from the other
Think firstCarry side AB on past B
The inside angle at B is 50°. What is the outside angle beside it?
Decide first, then press Next. Follow the dashed line on past corner B.
A triangle with corners A, B and C and inside angles 70°, 50° and 60°. Side AB is carried on past B as a dashed line, opening a shaded outside angle marked with a question mark. Then the flat line through B is drawn dark and a sentence reads 50° inside + 130° outside = 180°. Then a triangle with a square corner at B, 90° inside and 90° outside. The last step shows an inside angle of 115° at B.
See the whole ideathe big idea and the key word, in one place
The big idea
An exterior angle and the interior angle beside it add up to 180°.
Why 180°The carried-on side and the side it came from make one straight line. The inside and outside angles fill it between them.
Easy to get wrongCarry on the side that ends at the corner, keeping it straight. Any other line makes a different angle.
Key word
- exterior angle
- the angle outside a corner, between one side and the side before it carried on in a straight line
Ready to use it?
- Find an exterior angle from the interior angle beside it.