[h-man]

มาต่อกันดีกว่า

12. $ABCD$ is a convex quadrilateral. The perpendicular bisectors of $AB$ and $CD$ meet at $Y$. $X$ is a point inside $ABCD$ such that $∠ADX = ∠BCX < 90^o$ and $∠DAX = ∠CBX < 90^o$. Show that $∠AYB = 2 ∠ADX$.