Idea 16of 24Builds onA triangle's angles make 180°
Find the missing angle
How can you find an angle without measuring it?
Today you'll discover
- how to find a triangle's third angle
- the two steps to use
- how to use it on a ramp or a roof
Think firstTwo angles known, one unknown
Two angles are 40° and 75°. What is the third angle?
Decide first, then press Next. Look at the gap the known corners leave on the line.
A triangle with angles 40° and 75° and a question mark in its third corner. Its corners are laid side by side on a straight line, leaving a gap where the third goes, then the sentence 180° − 40° − 75° = 65°. Then a ramp leaning on a wall: 30° at the ground, a square corner at the wall, 60° at the top. The last step shows the end of a roof with two 35° angles at the bottom.
See the whole ideathe big idea and the key word, in one place
The big idea
To find the unknown angle, take the two known angles away from 180°.
In real lifeBuilders find missing angles in ramps, roofs and bridges this way, without climbing up to measure them.
Easy to get wrongTake away both known angles, not just one. 180° − 40° alone leaves out the 75°.
Key word
- unknown angle
- an angle whose size you work out from the angles you know
Ready to use it?
- Find a triangle's third angle from the other two.
- Solve angle problems about ramps and roofs.