使用积化和差:sin(α-β)sin(β-r)-cos(α-β)cos(r-β)=(-1/2)[cos(α-β+β-r)-cos(α-β-β+r)]-(1/2)[cos(α-β+r-β)+cos(α-β-r+β)]=(-1/2)[cos(α-r)-cos(α+r-2β)]-(1/2)[cos(α+r-2β)+cos(α-r)]=-cos(...