GA + 2GB + GC = 0
<==> A - G + 2B - 2G + C - G = 0
<==> A+2B+C = 4G
<==> G = (1/4) (A+2B+C) = (1/4) (1-2-2 ; -1-4+2) = (-3/4 ; - 3/4)
BD = BA + BC
<==> D-B = A-B+C-B
<==> D = A+C-B = (1-2+1 ; -1+2+2) = (0; 3)
B, D et G sont alignés
en effet,
BD = (1; 5) et BG = (1/4)(1; 5)