在等差数列中，若，则的值为   (      )

中国历史悠久，地大物博，很多景点是历史的再现和纪念。观察下面三幅图片，回答问题。

创客是指把非同凡响的创意转变为现实产品的人．他们不计得失，敢闯敢干，凝聚智慧，用青春拼写着梦想的蓝图．由此启示我们要（   ）

①追求名誉利益         ②具有创新精神

③树立进取意识         ④发扬团队精神

如图是2002年在北京召开的国际数学家大会的会徽，它由4个相同的直角三角形拼成，已知直角三角形的两条直角边长分别为3和4，则大正方形ABCD和小正方形EFGH的面积比是（　　）

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假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。每处错误仅涉及一个单词的增加、删除或修改。
增加：在缺词处加一个漏字符号（∧）并在其下面写出该加的词。
删除：把多余的词用斜线（\）划掉。
修改：在错的词下划一横线，并在该词下面写出修改后的词。
注意：1）每处错误及其修改均仅限一词；
2）只允许修改10处，多者（从第11处起）不计分。
    Last summer I go to America and studied at a language school. I had many wonderful experience, but I also had a sad one. One day, the school held party , where I invited to talk about Chongqing. After that they asked me a lot of things about China. But I couldn't explain them with English clearly. I felt sadly. I learned a lesson from this experience. I have already studied English for eight years, I can't use it very good. I must work hard to improve my spoken English so that I will not be able to communicate freely with foreigners. I hope I can be a bridge between China and others countries in the future.