cosC=2(cosC/2)^2-1,
(cosC/2)^2=(1+cosC)/2
(cosA/2)^2=(1+cosA)/2
sin(A+C)=sinB
sin(90°-B/2)=cosB/2
sinAcos^(C/2)+sinCcos^(A/2)=3/2sinB
sinA*[(1+cosC)/2]+sinC*[(1+cosA)/2]=3/2sinB
sinA+sinC+sin(A+C)=3sinB
sinA+sinC+sin(A+C)=3sinB
sinA+sinC=2sinB