已知函数sin（70°+α）=1/3,则cos（2α-40°）=
问题描述：
已知函数sin（70°+α）=1/3,则cos（2α-40°）=
答
cos(2α-40)
=cos(40-2α)
=-cos[180-(40-2α)]
=-cos(140+2α)
=-cos[2(70+α)]
=-[1-2sin²(70+α)]
=-7/9