A(x1,y1),B(x2,y2),C(x3,y3)
则重心坐标为
O=((x1+x2+x3)/3,(y1+y2+y3)/3)
OA=(x1-(x1+x2+x3)/3,y1-(y1+y2+y3)/3)
OB=(x2-(x1+x2+x3)/3,y2-(y1+y2+y3)/3)
OC=(x3-(x1+x2+x3)/3,y3-(y1+y2+y3)/3)
OA+OB+OC=0