已知cos(α-π/3)=2/3,求cos(2π/3+α)+tan^2(4π/3-α)=cos(π-π/3+α）+tan^2(π+π/3-α）
=-cos(α-π/3)+sin^2(α-π/3)/cos^2(α-π/3)=-2/3+5/4=7/12