Line Defined By Three Points (Ray) 
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The ray tool is mostly used during constructions. You would use it in situations where the line you are drawing is determined by a point that is NOT one of the end points.
When drawing in the angle bisector, you are likely to want to extend the line past the intersection which is a little difficult with the standard line tools. This is a great time to use the ray tool.
The ray tool is easier to use than to describe. To begin with, you draw a line to the intersection. This defines the angle of the line.
Now that you have defined the angle, if you move the mouse you should notice that the line changes length. It is always on the same angle  just the length has changed.
The line does not even need to be in the same direction.
Click again and your ray will appear.
If we select the ray you will see three selection squares.
