1. Know the angle of rotation.
2. Know the direction (either it is clockwise or counterclockwise)
3. Use the formula of the given angle to each point.
90 degree rotation
(x,y) → (-y,x)
-90 degree rotation
(x,y) → (-y,x)
180 degree rotation
(x,y) → (-x,-y)
270 degree rotation
(x,y) → (y,-x)
isn't -90 the same as 270?
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