Resposta :
cos 75 = cos (30+45)
cos (a+b) = cos a. cos b - sen a. sen b
cos (30+45) = cos 30 . cos 45 - sen 30 - sen 45
cos (30+45) = \/3/2 . \/2/2 - 1/2 . \/2/2
cos (30+45) = \/6/4 - \/2/4
cos (30+45) = (\/6-\/2)/4
cos (a+b) = cos a. cos b - sen a. sen b
cos (30+45) = cos 30 . cos 45 - sen 30 - sen 45
cos (30+45) = \/3/2 . \/2/2 - 1/2 . \/2/2
cos (30+45) = \/6/4 - \/2/4
cos (30+45) = (\/6-\/2)/4