Angle of Rotation
The fixed point is called the center of rotation .
The amount of rotation is called the angle of rotation and it is measured in degrees.
You can use a protractor to measure the specified angle counterclockwise.
Consider the figure below.
Here, is obtained by rotating by about the origin. Observe that both and are straight lines.
How many degrees has the been rotated counterclockwise to obtain the ?
Identify the corresponding vertices of the rotation.
The point of rotation is the origin, draw lines joining one of the points, say and it's image to the origin.
You can see that the lines form an angle of , in the counterclockwise direction.
Therefore, is obtained by rotating counterclockwise by about the origin.
So, the correct choice is .
Also note that the relation between the corresponding vertices is
which shows a counterclockwise rotation of
about the origin.