Quadrilaterals, tiling and the eight directions · Usually met around P4
Measuring angles in degrees
Read the ring that starts at 0°; line 180°, point 360°.
How to do it
Method
Read the ring that starts at 0°; line 180°, point 360°
On a line: 180° − 70° = 110°
Another way
Compare with a right angle
Bigger than a right angle: 130°, not 50°
In full
An angle's size is measured in degrees (°): a right angle is 90°, the angles on a straight line make 180°, and a whole turn round a point is 360°. To use a protractor, put its centre on the angle's corner and its 0° line along one arm, then see what the other arm points to. A protractor has two rings of numbers: read the one that starts at 0° on your arm. Arm on the 0° at the right: the inner ring; on the 0° at the left: the outer ring. Then estimate by eye: an angle smaller than a right angle must read less than 90°. Some angles can be found without measuring: angles on a straight line make 180°, so if one is 70°, the other is 180 − 70 = 110°; angles round a point make 360°. Another way: count the marks, each big step on a protractor is 10°.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
The angles in the picture are on a straight line. What is the angle marked ? in degrees?
Show the answer and working
Answer: 95°
Angles on a straight line make 180°: 180 − 85 = 95.
Example 2
The protractor shows ∠AOB = 130°.
True or false?
Show the answer and working
Answer: True
Reading from the 0° on OA's side, ∠AOB = 130°; first see whether it is bigger or smaller than a right angle.
Example 3
The angles in the picture are all round one point. What is the angle marked ? in degrees?
Show the answer and working
Answer: 185°
Add the known angles: 50 + 65 + 60 = 175; round a point is 360°: 360 − 175 = 185.
Practise it in the game →Quadrilaterals, tiling and the eight directions · Free to start, unlimited questions, no download