Modify the value of the rotation angle by moving the angle slider or typing the value in the text box.[br][br]Remember that:[br][br](a) the rotation angle is positive (negative) when OA is rotated in anti-clockwise (clockwise) direction.[br][br](b) sinθ=y/x, cosθ=x/r, tanθ=y/x (x ≠ 0), where r= sqrt (x^2+y^2)[br][br](c) r is always larger than 0[br][br](d) the CAST diagram. You can easily memorise it by saying "All Students Take Chinese." [br][br]Amended by Sit Hon Yip