cos(A/2-C/2)=2sin（B/2）cos(B/2)=coc[(180-A-C)]/2=sin(A/2+C/2)2cos(B/2)cos(A/2-C/2)=2sin（B/2）2cos(B/2)2sin(A/2+C/2)cos(A/2-C/2)=2sin（B/2）2cos(B/2)sinA+sinC=2sinB正弦定理a+c=2ba、b、c成等差数列...