sin(θ+π/3)=3/5
cos(θ+π/3)=-4/5
sinθ1/2+cosθ√3/2=3/5
sinθ√3/2+cosθ3/2=3√3/5
cosθ1/2-sinθ√3/2=-4/5
cosθ3/2+cosθ1/2=3√3/5-4/5
2cosθ=(3√3-4)/5
cosθ=(3√3-4)/10
sin(θ+π/3)=3/5 θ∈（π/6,2π/3）则cosθ=
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