tana+tanb=-5/3tana*tanb=-7/3sin(a+b)/cos(a-b)=(sinacosb+sinbcosa)/(cosacosb+sinasinb)=(tana+tanb)/(1+tanatanb)=-5/3/(1-7/3)=5/4tan(a+b)=(-5/3)/(1+7/3)=-1/2cos^2(a+b)=1/[1+tan^2(a+b)]=5/4