a/sinA=(b+c)/(sinB+sinC)正弦定理：a/sinA=b/sinB=c/sinC设a/sinA=b/sinB=c/sinC=k则b=sinB*k,c=sinC*k(b+c)/(sinB+sinC)=(sinB*k+sinC*k)/(sinB+sinC)=k(sinB+sinC)/(sinB+sinC)=k则a/sinA=(b+c)/(sinB+sinC)=k由...