cosb=-4/5,cosa=sin(a+b)=sina*cosb+cosa*sinb=-4/5 sina+3/5 cosa ,sina/cosa=-1/2,∴tan(a+b)=sin(a+b)/cos(a+b)=cosa/(cosa*cosb-sina*sinb) =cosa/(-4/5 cosa-3/5 sina) = -5/(4+3sin...