if cos2X=1/3 and 0<=2X<=pi, find cosX ...?
cos 2 X = 1/3cos 2 X = cos ² X - sin² Xcos ² X - sin ² X = 1/3+cos ² X + sin ² X = 1------------------------------2 cos ² X = 4/3 / : 2cos ² X = 2/3 cos X = √2/3 ( angle X is in the 1st quadrant )
