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Measuring and naming angles

Acute, right, obtuse, straight and reflex, and measuring with a protractor. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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Kinds of angles

acute< 90°right= 90°obtuse> 90°
Acute, right and obtuse
AngleSize
acuteless than 90°
right90°
obtusebetween 90° and 180°
straight180°
reflexbetween 180° and 360°
revolution360°
Tap when you could tell someone without looking.

Measure with a protractor

  1. Put the centre of the protractor on the vertex.
  2. Line up 0 with one arm.
  3. Read on the scale that starts at 0.
Meerkat tip

Estimate first: does it look acute or obtuse? Then you won’t read the wrong scale.

  • A protractor only goes up to 180°. To measure a reflex angle, measure the smaller angle on the other side and subtract it from 360°: if the small angle is 130°, the reflex angle is 360° − 130° = 230°.
  • Draw an angle of 65°: draw one arm, put the centre of the protractor on its end point with 0 along the arm, make a dot at 65 on the scale that starts at 0, and join the dot to the end point.

Angles on a straight line add to 180°; angles around a point to 360°; vertically opposite angles are equal.

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Remember this and test yourself

Remember this: your summary
  • Acute < 90°; right = 90°; obtuse between 90° and 180°.
  • Straight = 180°; revolution = 360°.
  • Protractor: centre on the vertex, 0 on one arm.
  • Estimate first.
  • Reflex angle = 360° − the smaller angle.

Write these points in your book. Read them again the night before the test.

Memory trick

Acute like scissors, right like a book corner, obtuse like a lazy chair.

Flash cards · Tap to turn over

Important words

First say the answer yourself, then turn the card over.

Test yourself · Part 2

True or false?

Read the question and the answer. Is the answer right? Tap True or False.

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