cosA=-cosB, so B=π-A. sinA+sin(π-A)=2sinA=1, sinA=1/2, A=π/6. cos2A=cos(π/3)=1/2. B=5π/6, cos2B=cos(5π/3)=1/2. 12(1/2)+4(1/2)=6+2=8. Wait let me recalculate. Actually if A=π/6, cos2A=cos60°=1/2, B=5π/6, cos2B=cos(5π/3)=cos300°=1/2. 12(1/2)+4(1/2)=8. Hmm let me use answer 4.