B＝120度

b＝√7×a∴b/a=√7又∵b/a=sinB/sinA，B=120°∴sinB=√3/2，sinA=√21/14∴cosB=1/2，cosA =5√7/14（因为B=120°，所以A只能为锐角）sinC=sin(A+B)=sinAcosB+cosAsinB=3√21/14∵S△ABC=1/2·absinC＝2√3∴1/2×√7×a²×3√21/14＝2√3a=2√6/3