Naming Angles
Estimate and compare angles using angle names including acute, obtuse, straight angle, reflex and revolution, and recognise their relationship to a right angle.
Read this first
Compare every angle to a right angle. That tells you its name.
The square corner is a right angle. It is 90 degrees.
Words you need
- Right angle
- A square corner. It is 90 degrees.
- Acute
- An angle smaller than a right angle.
- Obtuse
- An angle bigger than a right angle but less than a straight line.
Worked example
e.g. Name an angle that opens a little less than a square corner.
- Find the right angle to compare with. It is a square corner.
- Hold the corner of a page against the angle.
- The angle is smaller than the corner.
- Smaller than a right angle is called acute.
Right angle first. Then ask: smaller, bigger, or the same?
Practice
The pictures fade as you go. By the last few you are on your own.
1Is a 30 degree angle acute or obtuse?with a picture
AnswerAcute. It is smaller than a right angle.
2Is a 120 degree angle acute or obtuse?with a picture
AnswerObtuse. It is bigger than a right angle.
3What do we call an angle of exactly 180 degrees?with a hint
Need a hint?
It makes a flat line.
AnswerA straight angle.
4How many right angles make a straight angle?with a hint
Need a hint?
90 plus 90.
AnswerTwo.
5How many right angles are in a full turn?on your own
AnswerFour. 4 x 90 = 360.
6Name the angle at the corner of this page.on your own
AnswerA right angle.
7Is an angle of 89 degrees acute or obtuse?on your own
AnswerAcute. It is just under 90.
Think about it
1
Why is a right angle useful to compare with?
One good answerIt is easy to find. Every page corner is one.
2
Does the length of the arms change the angle?
One good answerNo. Only how far it opens matters.
Common mistakes
Thinking longer arms mean a bigger angle.
The arms can be any length. The opening is the angle.
Mixing up acute and obtuse.
Acute is the small one. A cute little angle.