（sin(A+B)）/(COS(A-B))
=(sinAcosB+cosAsinB)/(cosAcosB+sinAsinB)
=(tanA+tanB)/(1+tanAtanB)
=(-5/3)/(1-7/3)
=5/4
tan(A+B)=(-5/3)(1+7/3)=-1/2
cos^2(A+B)=1/[1+(tan(A+B))^2]=1/(1+1/4)=4/5