cos 255

since cos 255=cos (180+75)
=cos 75
=cos (45+30)
=(cos45)*(cos 30)-(sin45)*(sin 30) (ucing expansion of cos(c+d))
=(1/sqrt(2))*(sqrt(3)/2)-(1/sqrt(2))*(1/2)
=(1/2sqrt(2))*(sqrt(3)-1)