题目

It was a cold winter day. A woman drove up to the Rainbow Bridge tollbooth (收费站). “I’m paying for myself, and for the six cars behind me,” she said with a smile, handing over seven tickets. One after another, the next six drivers arriving at the tollbooth were informed, “Some lady up ahead already paid your fare.” It turned out that the woman, Natalie Smith, had read something on a friend’s refrigerator: “Practice random kindness and senseless acts of beauty.” The phrase impressed her so much that she copied it down. Judy Foreman spotted the same phrase on a wareh已知AB平行CD，AD，BC相交于点O，若OA=2，OD=4，AB=3，求CD的长度．