Question 13
$E$ lies on side $AB$ and $F$ on side $CD$ of square $ABCD$. When the square is folded along $EF$, the new position $A^\prime$ of $A$ lies on $BC$. Let $D^\prime$ be the new position of $D$, and let $G$ be the intersection of $CF$ and $A^\prime D^\prime$. Prove $A^\prime E+FG=A^\prime G$.
