∵cos(π/3-θ)=cosπ/3cosθ+sinπ/3sinθ)=1/2cosθ+√3/2sinθ∵cos(π/3-θ)=-3/5∴sin²θ=(-6/5-cosθ)²/3=1/3*cos²θ+4/5*cosθ+12/25∵sin²θ+cos²θ=1∴4/3*cos²θ+4/5*cosθ-13/25=0∴cosθ=(-3±4√3)/10∵-π/2<θ<0∴cosθ=(-3+4√3)/10
