sinA：sinB：sinC=3:2:4由正弦定理,化为边的形式a:b:c=3:2:4设a=3k b=2k c=4k由余弦定理cosC=(a²+b²-c²)/(2ab)=(9k²+4k²-16k²)/(2*3k*2k)=(-3)/12=-1/4