Angles · Usually met around P6
The protractor and naming angles
Read the scale that starts at 0°, then estimate.
How to do it
Method
Read the scale that starts at 0°, then estimate
Arm on the left, read the outer scale: ∠AOB = 50°
Another way
Take one reading from the other
130 − 40 = 90
In full
An angle's size is measured in degrees (°): split a full turn into 360 equal parts, each 1°. A right angle is 90°, a straight angle 180° and a full turn 360°. Less than 90° is acute, between 90° and 180° obtuse, more than 180° but less than 360° reflex. To measure, put the protractor's centre on the angle's corner and its 0° line along one arm, then read from the scale that starts at 0° on that arm: the arm on the right, the inner scale; on the left, the outer one. Another way: if neither arm is on 0°, take one reading from the other: 130 − 40 = 90. Then estimate by eye: an angle smaller than a right angle must read less than 90°. An angle is named with three letters, its corner in the middle: the angle at B is ∠ABC or ∠CBA. To draw one, draw an arm, put the centre on its end, dot the size you want and join it up.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
A protractor measures the angle. How many degrees is ∠AOB?
Show the answer and working
Answer: 125°
OA is on the 0° at the right, so read the inner scale: OB points at 125, ∠AOB = 125°.
Example 2
A protractor measures the angle. How many degrees is ∠AOB?
Show the answer and working
Answer: 110°
OA is on the 0° at the left, so read the outer scale: OB points at 110, ∠AOB = 110° (the inner 70 belongs to the other angle).
Example 3
Drawing a 145° angle with a protractor: the centre on the corner, one arm on the inner 0°, then a dot at 35° on the inner scale, joined to the corner.
True or false?
Show the answer and working
Answer: False
With the arm on the inner 0°, count to 145° on that same scale.
Practise it in the game →Angles · Free to start, unlimited questions, no download