An angle is the amount of turn between two arms. Compare it with a right angle (a square corner, 90°) and a straight line (180°). Smaller than a right angle is acute. Exactly a square corner is a right angle. Between a right angle and a straight line is obtuse. A straight line is a straight angle. More than a straight line is reflex: the arc goes the long way round. Arm length never changes the type, and neither does turning the picture.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- acute
- less than 90°
- obtuse
- between 90° and 180°
- reflex
- between 180° and 360°
Worked examples
What type of angle is 35°?
- 1.35° is less than 90°.
- 2.Less than a right angle is acute.
= acute
What type of angle is 250°?
- 1.250° is more than 180°, a straight line.
- 2.More than a straight angle is reflex.
= reflex
An angle is 40° bigger than a right angle. What type is it?
- 1.A right angle is 90°, so the angle is 90 + 40 = 130°.
- 2.130° is between 90° and 180°, so it is obtuse.
= obtuse
For parents
Method: Use the corner of a book as a right-angle tester.
What to say: Is it smaller than the corner of the book, exactly the same, or bigger? Is it more than a straight line?
Common mistakes:
- Mixes up the words acute and obtuse.
- Looks at the smaller gap between the arms instead of the marked reflex angle.
- Calls every angle bigger than a right angle obtuse, including straight and reflex angles.
- Mixes up a right angle (90°, a square corner) with a straight angle (180°, a straight line).