解,根据正弦定理a/sinA=b/sinB=c/sinC=2R得sinA=a/2R,sinB=b/2R,sinC=c/2R已知sinA：sinB：sinC=3:2:4得a：/b：c=3:2:4令a=3t,b=2t,c=4t则cosC=（a² + b² - c²）/2ab=-3t² /12t²=-1/4...