Algebraic Method to find the rotation about a point
This material shows an algebraic method to find the rotation (90, 180, 270 anticlockwise) of a point A about any point C which is not the origin. Consider a point A rotated about the center C. Step 1: We change A to A1=A-C Step 2: We apply the rule for rotation of point A1 about origin to get A2 (a) 90 anticlockwise (x,y)->(-y,x) (b) 180 anticlockwise (x,y)->(-x,-y) (c) 270 anticlockwise (90 clockwise) (x,y)->(y,-x) Step 3: We add C to get A3=A2+C
Finally, we observe that A, A1, A2 and A3 form a parallelogram. By moving the slider, we can change the angle of rotation. Move the point A or C.