设a-(b/2)=α ,(a/2)-b=β则(a+b)/2=α-β∴sin(a+b)/2=sin(α-β)=sinαcosβ-cosαsinβ∵π/2∴π/2<α<π,0<β<π/2, sinα=4√5/9,cosβ=√5/3.∴sin(a+b)/2=4√5/9*√5/3-(-1/9)*2/3=22/27