cosC=a�0�5+b�0�5-c�0�5/2ab,乘以2a得：a�0�5+b�0�5-c�0�5=2b�0�5-bc,所以整理得：bc=b�0�5+c�0�5-a�0�5.所以cosA=1/2,因为A为内角A=60
