A:若x=y,则x﹣a=y+a
B:若 ac = bc ,则 ac2 = bc2
C:若ac2=bc2,则a=b
D:若x=y,则 xa+2 = ya+2
已知向量AB→=cos129°,sin120°BC→=cos30°,sin45°,则△ABC的形状为( )