cos2αcos2β-cos^2(α-β)=cos2αcos2β-1/2[1+co2(α-β)]=cos2αcos2β-1/2[1+cos2αcos2β+sin2αsin2β]=-1/2+1/2[cos2αcos2β-sin2αsin2β]=-1/2+1/2cos2(α+β)=-1/2+1/2[(1-tan²(α+β)/(1+tan²...