nilai dari cos 855 =

cos885 = cos165

cos 165 = cos(120+45) = cos120cos45 - sin120sin45
cos120= cos(2x60) = cos^2(60) - sin^2(60) = 1/4 - 3/4 = -1/2
sin120 = sin(2x60) = 2sin60cos60 = 2 x 1/2 x[tex] \sqrt{3} [/tex]/2 =[tex] \sqrt{3} [/tex]/4

maka cos 165 = -1/2 x [tex] \sqrt{2} [/tex]/2 - [tex] \sqrt{3} [/tex]/4 x [tex] \sqrt{2} [/tex]/2  = -[tex] \sqrt{2} [/tex]/4 - [tex] \sqrt{3} [/tex] /16

sorry kalo salah