cosC/cosB=（2a-c）/b=(2sinA-sinC)/sinBcoscsinB=2sinAcosB-sinCcosBcoscsinB+sinCcosB-2sinAcosB=0sin(B+C)-2sinAcosB=0而B+C=180°-A所以：sinA-2sinAcosB=0,sinA(1-2cosB)=0cosB=1/2所以∠B=60°很高兴为您解答,...