Naming an Angle

Lines

Let me show you different kinds of lines and how we label them.  

(slide the slider to change line types and move the points around)  Watch video

Line Definitions

Line: passes through two points and extends to infinity in both directions, notated 

Line segment (interval): starts at a point and ends at a point, notated 

Ray: starts at a point and extends through a second point to infinity, notated 

Naming angles

Angles inside shapes

This quadrilateral has 4 vertices. They are labelled $A$A, $B$B, $C$C and $D$D.  

The edges of the shape would be labelled like line segments.  

We can call them AB, BC, CD, DA . 

The angle marked has apex (the pointy bit of the angle) at $B$B.

We label an angle by moving from one point on the line segment, through the APEX to a point on the the other line segment.  Thus the shaded angle is called $\angle ABC$∠ABC or $\angle CBA$∠CBA. See the angle symbol at the front, this is important it tells us you are talking about an angle.

Can you see where $\angle ADC$∠ADC is ?  What about $\angle DCB$∠DCB ?

Angles in lines

Lines, Line Segments (intervals) and Rays that cross create angles.

Have a play with this applet, you can create lines, line segments and rays that cross each other.  Change the slider to change the type of lines, move the points to place you like, then slide the slider to show the angles that are created. Watch video

Worked Example

Question 1

In the following, state the vertex and name the angle.