，当且仅当“”时，等号成立．示例：当时，求的最小值．
解：，当，即时，的最小值为．
尝试：当时，求的最小值．
问题解决：随着人们生活水平的快速提高，小轿车已成为越来越多家庭的交通工具，假设某种小轿车的购车费用为万元，每年应缴保险费等各类费用共计万元，年的保养、维护费用总和为万元．问这种小轿车使用多少年报废最合算（即：使用多少年的年平均费用最少，年平均费用所有费用之和年数）？最少年平均费用为多少万元？

    William Lindesay, a British traveler and writer, has been exploring the Great Wall—one of the human’s most wonderful building projects. Not even a Chinese knows the Great Wall of China as well as he does.
    When, as a schoolboy, Lindesay announced his plan to explore the Great Wall, nobody took him seriously. But Lindesay achieved his goal, by spending 25 years on the Great Wall and has become one of the world’s most famous experts on its rich history. Lindesay now lives and works in Beijing. This year his new book has come out, The Great Wall Explained, with a lot of photographs and articles.
    Running on the Great Wall was a brave trip and Lindesay had already run 3,000 kilometers along it! During those years, he got to many villages along the way. He was often helped by friendly villagers, although they didn’t quite understand why the Englishman would have such interest in the Great Wall.

    I had hidden the long black bag in the garage for t he past two days. Tonight, in the dark, I would finally take it away. If everything went OK, it would be done by morning. But I had to be sure no one, not even my wife, saw me.
    It was more difficult to get away from the party than I expected. I said that the children were tired and needed to get to bed. It was partly true. But the main reason for leaving was that I wanted to complete my plan.
    First I had to get the kids to sleep. That was never easy at any time and tonight it was even more difficult. The eldest one wanted to know why we left the party early. I told him 10:30 pm was not early. As usual, I read them a bed time story, but I had to stop myself reading too quickly or they would learn my secret.
    Silence finally came, and feeling like a thief in the night, I went out of the house and into the garage. Taking one end, I dragged the bag out from its hid ing place and took it into the garden. I was worried that my neighbours might see me and call the police.
    It was already eleven o'clock. By the light of the moon I started working, trying to make as little noise as possible. And that was not easy with a saw (锯子) and a hammer.
    After much effort, my work was finally completed. I looked down at my watch. It was exactly 4:00 am. Then I went inside to have a shower and get a few hours of sleep. It wouldn't be long now before the kids would rush into my room and wake me up to tell me about the new tree-house Father Christmas had brought them this year.

下图所展示的民族舞蹈流传于（    ）