2. Given $H$ and $O$ are the orthocenter and circumcenter of acute $\triangle ABC$ respectively, $OD \perp BC$, with the foot of the perpendicular being $D$, then $AH: OD=$
2.2:1. Extend $B O$ to intersect the circumcircle of $\triangle A B C$ at $E$, and prove that quadrilateral $A H C E$ is a parallelogram.