I found a theorem, I named it is : Viet Nam Hexagon theorem follow:

Let hexagon ABCDEF. AC meets BD at G, BE meets CF at H, AE meets DF at I. Then G,H,I are collinear <=> A,B,C,D,E,F lie on a conic <=> $\frac{\overline{GB}}{\overline{GD}}*\frac{\overline{ID}}{\overline{IF}}*\frac{\overline{HF}}{\overline{HC}}*\frac{\overline{G...$