sinα=4/5
cosα= √{1-sin²α} = √{1-16/25} = √9/25 = 3/5
sin 2α = 2 sin α · cos α = 2*4/5 * 3/5 = 24/25
cos 2α = cos² α - sin² α = 9/25 - 16/25 = -7/25
