Question

cos 75º = cos (45º + 30º), determine o valor de cos 2 985º

Answer 1

2985 =8*360+105

cos 2985 =cos 105 = cos (180-75) =cos 180*cos75 +sen180*sen75 =-cos75

cos 75=cos45*cos30 -sen45*sen30 =√2/2 * √3/2 - √2/2 * 1/2

cos 75 = √6/2 -√2/4

cos 2985 =- cos 75 = - [ √6/2 -√2/4] = √2/4 -√6/4