(a²+b²-c²)²-（ab)²=0
[(a²+b²-c²)+（ab)][(a²+b²-c²)-（ab)]=0
(a²+b²-c²)+（ab)=0或(a²+b²-c²)-（ab)=0
cosC=(a²+b²-c²)/2ab=-1/2 或cosC=(a²+b²-c²)/（2ab)=1/2
C=60°或C=120°