问题
选择题
在正方体ABCD-A1B1C1D1中，M为DD1的中点，O为底面ABCD的中心，P为棱A1B1上任意一点，则直线OP与直线AM所成的角是（ ）
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答案
解：（特殊位置法）将P点取为A1，
作OE⊥AD于E，连接A1E，
则A1E为OA1在平面AD1内的射影，
又AM⊥A1E，
∴AM⊥OA1，即AM与OP成90°角．
故选D．
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在正方体ABCD-A1B1C1D1中，M为DD1的中点，O为底面ABCD的中心，P为棱A1B1上任意一点，则直线OP与直线AM所成的角是（ ）
|
解：（特殊位置法）将P点取为A1，
作OE⊥AD于E，连接A1E，
则A1E为OA1在平面AD1内的射影，
又AM⊥A1E，
∴AM⊥OA1，即AM与OP成90°角．
故选D．
There are many kinds of friends. Some are always 26 you, but don't understand you. Some say only a few words to you, but understand you. Many people will step in your life, but only 27 friends leave footprints (脚印). I shall always think of the autumn and the girl with the 28 . It was the golden season. I could see the yellow leaves 29 in the cool wind. Autumn is a harvest（丰收）season and life is uninteresting. The free days always let 30 down. But one day, the sound of a violin came into my ears. I was so surprised that I jumped to see what it was. A young girl, standing in the wind, was 31 in playing her violin. I had 32 seen her before. The music was so nice that I didn't know I had been 33 there for so long. Every day I went to watch her performance. I was the only listener. The autumn seemed no longer lonely and life became 34 . I thought we were already good friends, 35 we didn't know each other. Autumn was nearly over. One day, when I was listening carefully, the sound suddenly 36 . To my surprise, the girl came over to me. “You must like violin.” she said. “Yes. And you play very well. Why did you stop?” I asked. 37 , a sad expression appeared on her face and I could feel something unusual. “I came here to see my grandmother, but now I must leave. I once played very badly. It was your listening every day that 38 me.” she said. “ 39 , it was your playing that gave me a meaningful autumn,” I answered, “Let's be friends.” The girl smiled, and so did I. I never heard her play again in my life. But I will always remember the girl. She is like a dream—so short, so bright, like a star giving off so much light that it makes the 40 beautiful.
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