Measure the size of angles accurately using a protractor | Maths

Starter quiz

Match the shape to its properties.
- Equilateral triangle
  
  ⇔
  
  A triangle with three equal sides and three equal vertices. ✓
- Isosceles triangle
  
  ⇔
  
  A triangle with two equal sides and two equal vertices. ✓
- Scalene triangle
  
  ⇔
  
  A triangle with no equal sides or vertices. ✓
Which of these is the best estimate for the angle shown?
- 150°
- 180°
- 220° ✓
- 300°
The image is not to scale. What is the value of missing angle a? a = ______°
- '83' ✓
Which of these is the best estimate for the angle shown?
- 85°
- 100°
- 175° ✓
- 180°
The image is not to scale. Calculate missing angle a. a = ______°
- '68' ✓
The image is not to scale. Complete the sentence. I estimate that the size of angle a is ___ degrees.
- 85
- 90
- 95 ✓
- 100 ✓

Exit quiz

Which image shows the protractor placed correctly to measure the angle?
The size of the angle is ___.
- 155°
- 140°
- 55°
- 35° ✓
Read the protractor scale to measure the angle. ______°
- '80°' ✓
Read the protractor scale to measure the angle. ______°
- 155°
- 145°
- 45°
- 55° ✓
Sofia measured the obtuse angle in order to find the missing angle a. What is her next step?
- Subtract 115 from 180
- Subtract 115 from 360 ✓
- Double 115
- Guess the size of a based on the angle of 115°
Jun measured the acute angle in order to find the missing angle a. Tick what his next steps could be.
- Subtract 43 from 180 ✓
- Subtract 43 from 360
- Add 43 with 180, then subtract from 360
- Subtract 180 then subtract 43 from 360

Worksheet

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Presentation

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Lesson Details

Key learning points

A protractor is an angle measurer.
Protractors have two scales from 0 to 180 degrees.
Start from zero when reading the scale on a protractor.
Line the protractor up with one of the lines forming the angle.

Common misconception

Pupils incorrectly place the protractor on the angle and therefore cannot measure the angle. Pupils use the incorrect scale to read the angle.

Stress the importance of using the middle spot on the protractor to line this up with the vertex of the angle as well as placing one of the lines on a zero line, ensuring the remaining line crosses the scale of the protractor.

Keywords

Angle - An angle is a measure of turn. It shows how far something has rotated. It is often displayed as the rotation between two line segments.
Point - A point is an exact location. It has no size, only position.
Vertex - A vertex is a point where two or more lines meet.
Scale - A scale is a number line with equal divisions for equal values.