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### Course: 4th grade > Unit 11

Lesson 2: Angle introduction# Angle types review

Review the following types of angles: acute, right, obtuse, and straight. Identify and draw types of angles in some practice problems.

### Summary of angle sizes

Angle type | Angle size (in degrees) |
---|---|

Acute | Between |

Right | Exactly |

Obtuse | Between |

Straight | Exactly |

Reflex | Between |

Complete | Exactly |

### Right angles

A right angle is a ${90}^{\circ}$ angle. A right angle is in the shape of a perfect corner, like the corner of a rectangular sheet of paper. Below is an example of a right angle.

### Straight angles

A straight angle is a ${180}^{\circ}$ angle. A straight angle looks like a straight line. Below is an example of a straight angle.

### Acute angles

An acute angle is an angle whose degree measure is less than ${90}^{\circ}$ . Below is an example of an acute angle.

When we compare an acute angle to a ${\text{right angle}}$ , we can see that an acute angle is less than ${90}^{\circ}$ .

### Obtuse angles

An obtuse angle is an angle whose degree measure is greater than ${90}^{\circ}$ but less than ${180}^{\circ}$ . Below is an example of an obtuse angle.

When we compare an obtuse angle to a ${\text{right angle}}$ , we can see that an obtuse angle is greater than ${90}^{\circ}$ .

When we compare an obtuse angle to a ${\text{straight angle}}$ , we can see that an obtuse angle is less than ${180}^{\circ}$ .

*Want to learn more about angle types? Check out this video.*

### Reflex angles

A reflex angle is an angle whose degree measure is greater than ${180}^{\circ}$ but less than ${360}^{\circ}$ . Below is an example of a reflex angle.

When we compare a reflex angle to a ${\text{straight angle}}$ , we can see that a reflex angle is greater than ${180}^{\circ}$ .

### Complete angles

A complete angle is a ${360}^{\circ}$ angle. A complete angle has both rays pointing the exact same direction. That is because we've turned a full circle from the first ray. Below is an example of a complete angle.

### Practice set 2: Angles in diagrams and images

*Want to try more problems like this? Check out this exercise.*

## Want to join the conversation?

- is there any angle that could be greater than 180 degrees?(74 votes)
- no. After 180 degrees it starts bending backwards, and it would be a acute angle again. A circle equals 360 degrees though.(9 votes)

- the last one confused me a little bit(27 votes)
- Yeah it was very confusing(24 votes)

- How is this "Right". Saying that everything around me is ATLEAST Right, Obtuse or strait? How and why? And if you go on drawing right, acute and obtuse angles then they don't let you make one how ever you want on the dots?(17 votes)
- Is there an angle over 720 degrees?(14 votes)
- Yes, angles can have any angle measure. But since there's only 360 degrees in a circle, we usually don't write angles any bigger than 360. A 720 degree angle is the same as a 360 degree angle.(11 votes)

- Is a straight angle a 180 degree angle(9 votes)
- yes because 180 degree angle is straight like a straight angle(8 votes)

- hi am a new person for khan academy(10 votes)
- who likes math cuz i do(10 votes)
- I'm with you. I like math.(5 votes)

- How am I supposed to know what angle to make if it dosen't even tell me(6 votes)
- It already said what angle you need to make, which is a straight angle (180 degrees).(8 votes)

- I HAVE A JOKE how do u keep warm in the winter go to a corner its all was 90 digress.(8 votes)
- u spelled degrees wrong(2 votes)

- 9+9 =18 but you add a zero

to 18 90+90 = 180(7 votes)- so true and right DeAndre.(3 votes)