根据正弦定理有：a/c=sinA/sinC因此：(2a-C)/C=tanB/tanC2a/c-1=tanB/tanC2sinA/sinC -1=sinBcosC/cosBsinC2sinAcosB-cosBsinC=sinBcosC2sinAcosB=sinBcosC+cosBcosC2sinAcosB=sin(B+C)又∵A+B+C=180°∴sin(B+C)=si...