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Grade 4 · Maths · Lessons
Angles that add up
📐angle➕add➖take away↔️straight line
Angles that add.
The big ideaThe parts of an angle add up to the whole angle.

How it works

The parts of an angle add up to the whole angle.

A straight line is 180 degrees. A full turn is 360 degrees.

To find a missing part, take the part you know from the whole.

Whole: 180• part: 120• other part: ?Find it• 180 − 120 = 60• check: 120 + 60 = 180
Parts of a straight line.

Example 1: A straight line

  1. A straight line is 180 degrees. One part is 120. Find the other part.
  2. The whole is 180.
  3. 180 − 120 = 60.
  4. So the answer is 60.

Example 2: A full turn

  1. A full turn is 360 degrees. One part is 225. Find the other part.
  2. The whole is 360.
  3. 360 − 225 = 135.
  4. So the answer is 135.

Example 3: A small part

  1. A full turn is 360 degrees. One part is 40. Find the other part.
  2. The whole is 360.
  3. 360 − 40 = 320.
  4. So the answer is 320.

Words to know

straight line
An angle of 180 degrees.
whole
All of the angle.

Watch out

Try it

  1. A full turn is 360 degrees. One part is 330. Find the other part.
  2. A full turn is 360 degrees. One part is 120. Find the other part.
  3. A full turn is 360 degrees. Two parts are 120 and 60. Find the third.
Answers: 1. 30 · 2. 240 · 3. 180
For grown-upsOpen a book to different angles and talk about the part you can't see.
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