已知sin(70°+α)=1/3,则cos(2α-40°)= _ .
问题描述:
已知sin(70°+α)=
,则cos(2α-40°)= ___ .1 3
答
∵sin(70°+α)=
,∴cos(20°-α)=1 3
.1 3
∴cos(2α-40°)=2cos2(α-20°)-1=2×(
)2-1=-1 3
.7 9
故答案为-
.7 9