依题设,得 sinθ+sin(θ-π/3)=3/2 sinθsin(θ-π/3)=m/2
sin(θ-π/3)=sin(θ)cos(π/3)-cosθsin(π/3)=(1/2)sinθ-(√3/2)cosθ
所以sinθ+sin(θ-π/3)=(3/2)sinθ-(√3/2)cosθ=3/2
sinθ=(1/√3)cosθ+1
1=sin²θ+cos²θ=[(1/√3)cosθ+1]²+cos²θ=（4/3）cos²θ+(2/√3)cosθ+1
所以（4/3）cos²θ+(2/√3)cosθ=0 得 cosθ=0或cosθ=-(2/√3)/(4/3)=-√3/2
θ是钝角θ,所以cosθ