c / sinC = b / sinB = a / sinA sinB =sinC / 2b > c 所以B为钝角,cosB = -（1-sinC*sinC/4）^0.5a = c*sinA / sinC=c*sin(B+C) / sinC=c*（sinB*cosC+sinC*cosB)/sinC带入数值计算即可