a/sinA=b/sinB=c/sinC所以sinA：sinB：sinC=a:b:c=7:8:13所以cosC=(a²+b²-c²)/2ab=-56/112=-1/2C=120度
因为sinA：sinB：sinC=7:8:13所以a:b:c=7:8:13设a=7k b=8k c=13kcosc=(a²+b²-c²)/（2ab）=-1/2c=120°谢谢采纳···
