已知sin(30°+α)=1/3,求cos(120°-2α)的值_数学解答

已知sin(30°+α)=1/3,求cos(120°-2α)的值

人气：105 ℃　时间：2020-04-08 07:58:55

解答

cos(120°-2α)
=cos(180°-60°-2α)
=－cos(60°+2α)
=－cos2(30°+α)
=－(1-2(sin(30°+α))^2)
=－(1-2(1/3)^2)
=－7/9