Cos 45+ cos 165 + cos 285 adalah??

cos(165)=cos(180-15)=-cos(15)
cos(285)=cos(270+15)=-sin(15)

misal u=cos(15)+sin(15)
u^2=cos^2(15)+sin^2(15)+2cos(15)sin(15)
u^2=1+sin(30)
u^2=1+1/2=3/2
u=akar(6)/2

cos(45)+cos(165)+cos(285)=cos(45)-cos(15)-sin(15)=cos(45)-u=akar(2)/2-akar(6)/2