a=sin5π/7=sin2π/7b=cos(-2π/7)=cos2π/7c=tan9π/7=tan2π/72π/7属于(π/4,π/2)sin2π/7>cos2π/7 tan2π/7=sin2π/7/cos2π/7>1tan2π/7>sin2π/7>cos2π/7