Study Guide

Basic Geometry - Angles

Angles

Not to be confused with angels, angles are the pointed corners of shapes. Angles can be named three different ways.

This angle could be named any of these ways:

  • ABC (counterclockwise)
  • CBA (clockwise)
  • B (just the vertex)

Angle ABC

Look Out: when naming an angle using three letters, the vertex must always be the middle letter!

Types of Angles

NameDescriptionExample
Acute AngleAn angle less that 90°.

Lame joke: this angle is so small and a-cute!
Angle DEF
Obtuse AngleAn angle greater than 90°, but less than 180°.Angle GHI
Right AngleAn angle that is exactly 90°.

This is commonly denoted with a small box in the corner.
Angle KLM
Straight AngleA 180° angle.

It's - you guessed it - a straight line.
ABC line
Supplementary Angles and Linear PairsAngles that add up to 180°.

Supplementary Angles RST/TSU are supplementary angles.

If the two angles are adjacent (next to each other) and add up to 180°, they are a linear pair. 

Supplementary Angles RST/TSU are also a linear pair. 
RSU
Complementary AnglesAngles that add up to 90°.

NOQ and QOP are complementary.
NOP
Vertical AnglesWhen two lines intersect, opposite angles are called "vertical angles".
Vertical angles are congruent.

angle 1 is equal and congruent to angle 3
angle 2 is equal and congruent to angle 4
1234

See the squiggly line over the equal sign above? This symbol means congruent. Oh, and btw, congruent angles have the same measurement.