稍等,符号不好打上式=[1-cos(B+C)]/2+(2cos²A-1)
=1+cosA +2cos²A-1
=1/3+2/9
=5/9答案是－1/9诶。。上式=[1-cos(B+C)]/2+(2cos²A-1)
=(1+cosA)/2 +2cos²A-1
=2/3+2/9-1
=-1/9