26. Let $A A^{\prime}$ and $B B^{\prime}$ be two line segments which are perpendicular to $A^{\prime} B^{\prime}$. The lengths of $A A^{\prime}$, $B B^{\prime}$ and $A^{\prime} B^{\prime}$ are 680,2000 and 2010 respectively. Find the minimal length of $A X+X B$ where $X$ is a point between $A^{\prime}$ and $B^{\prime}$.
26. Ans: 3350 .
Take the reflection with respect to $A^{\prime} B^{\prime}$. Let $A^{\prime \prime}$ be the image of $A$. Then the minimal length is equal to the length of $A^{\prime \prime} B=\sqrt{2010^{2}+(2000+680)^{2}}=3350$.