由正弦定理a/sinA=b/sinB=c/sinC∵sinA：sinB:sinC=2:3:4.∴ a:b:c=2:3:4设 a=2t,b=3t,c=4t由余弦定理cosC=(a²+b²-c²)/(2ab)=(4t²+9t²-16t²)/(2*2t*3t)=-3t²/(1...