用动词的适当形式填空。（10分）
Yesterday Tom had no work to do after lunch. So his father asked him ______(go) to the bookstore to buy a few books. He _____(hurry)to the bus stop and arrived there at three. When he ____(wait) for a bus, he saw a little dog. It ___________(go) across the street. Just then a car ____(come), driving fast. But it ______(be) too late. The dog _______(fall) down and (not move)______
“Poor dog!” he cried .But look! Suddenly it ________(stand) up and went away.
He was very glad ______(find) it was safe.
，则该二阶方程为______．