Geometry · Angles · 1.1

Types of angles.

Angles are measured in degrees. An acute angle is less than 90°, a right angle is exactly 90° (a square corner), an obtuse angle is between 90° and 180°, and a straight angle is exactly 180° (a straight line).

01 / THE IDEA

See it work.

Watch the method

Types of angles

Acute < 90°, right = 90°, obtuse 90°–180°, straight = 180°.

1 / 2
1

Area of a rectangle

Area is length times width.

l×wl \times w
2

Multiply

Multiply the two dimensions.

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— / WHY IT WORKS

The idea underneath.

Compare to 90° and 180°

The two landmark angles, 90° and 180°, split every angle into a type.

Why it works

A right angle is a perfect corner.

03 / WORKED EXAMPLES

From easy to tricky.

EX 1easy

What type of angle is 81°?

1

Answer

Acute

EX 2medium

What type of angle is 90°?

1

Answer

Right

EX 3hard

What type of angle is 180°?

1

Answer

Straight

04 / COMMON TRAPS

Where students slip.

Trap

Calling 90° acute.

Fix

90° is exactly right, not acute.

Trap

Confusing obtuse and straight.

Fix

Straight is exactly 180°; obtuse is less.

05 / QUIZ

Test yourself.

The Test

12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.

0/12Answered
How to answerType just the value. Formatting won't trip you up — we ignore spaces, capital letters, commas, and subscript style, so t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).
Core

Core (4)

C1

What type of angle is 56°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
C2

What type of angle is 50°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
C3

What type of angle is 20°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
C4

What type of angle is 37°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
Apply

Apply (4)

A1

What type of angle is 90°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
A2

What type of angle is 90°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
A3

What type of angle is 180°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
A4

What type of angle is 180°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
Extension

Extension (4)

E1

What type of angle is 90°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
E2

What type of angle is 180°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
E3

What type of angle is 90°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.
E4

What type of angle is 41°?

Need a hint?
  • Compare the angle to 90°.
  • Then to 180°.