猎狗能以最大速度v1=10m/s持续地奔跑，野兔只能以最大速度v2=10m/s的速度持续奔跑．一只野兔在离洞窟s1=200m处的草地上玩耍，被猎狗发现后径直朝野兔追来．兔子发现猎狗时，与猎狗相距s2=60m，兔子立即掉头跑向洞窟．设猎狗、野兔、洞窟总在同一直线上，野兔的加速过程是匀加速直线运动．求：野兔的加速度至少要多大才能保证安全回到洞窟．
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| In the doorway of my home, I looked closely at my 23-year-old son, Daniel. In a few hours he would be flying to France to 1 a different life. It was a transitional (过渡的) time in Daniel's life. I wanted to 2 him some words of significance. But nothing came from my lips, and this was not the 3 time I had let such moments pass. When Daniel was five, I took him to the bus stop on his first day of kindergarten. He asked, "What is it going to be like, Dad? Can I do it?" Then he walked 4 the steps of the bus and disappeared inside. The bus drove away and I said nothing. A decade later, a similar 5 played itself out. I drove him to college. As I started to leave, I tried to think of something to say to give him 6 and confidence as he started this new stage of life. Again, words 7 me. Now, as I stood before him, I thought of those 8 opportunities. How many times have I let such moments 9 ? I don't find a quiet moment to tell him what he has 10 to me. Or what he might 11 to face in the years ahead. Maybe I thought it was not necessary to say anything. What does it matter in the course of a lifetime if a father never tells a son what he really thinks of him? 12 as I stood before Daniel, I knew that it did matter. My father and I loved each other. Yet I always 13 never hearing him put his 14 into words. Now I could feel my palms sweat and my throat tighten. Why is it so 15 to tell a son somethin from the heart? My mouth turned dry, and I knew I would be able to get out only a few words clearly. "Daniel," I said, "if I could have picked, I would have picked you." That's all I could say. He hugged me. For a moment, the world 16 , and there were just Daniel and me. He was saying something, but tears misted my eyes, and I couldn't understand what he was saying. All I was 17 of was the stubble (短须) on his chin as his face pressed 18 mine. What I had said to Daniel was 19 . It was nothing. And yet, it was 20 . | ||||
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