图所示电路中，电源两端电压保持不变，滑动变阻器的最大阻值为R。当开关S、S₁闭合，S₂断开，滑动变阻器的滑片P移到A端时，电阻R₂的电功率为P₂，电流表的示数为I₁；当开关S、S₂闭合，S₁断开时（滑片P位置不变），电阻R₂的电功率为P₂'，电流表的示数为I₂，且P₂:P₂'＝16:25；当开关S闭合，S₁和S₂都断开，滑动变阻器的滑片P移到某点C（图中未画出）时，滑动变阻器接入电路的电阻为R_C，电压表V₁的示数为U₁，电压表V₂的示数为U₂，已知U₁:U₂＝1:2，这时R₂的电功率为9W，R_C的电功率为3W。

（1）求I₁与I₂的比值；
（2）求R_A与R_C的比值；
（3）若通过闭合或断开开关及移动滑动变阻器的滑片P，使电路的电功率达到最大，求此时的电功率P_m。

一个多项式与x²-2x+1的和是3x-2，则这个多项式为（　　）

“对于历史上任何一个闯入并扎根于农耕文化圈的游牧民族来说，接受同化是他们的终极命运”。北魏孝文帝顺应历史时代潮流，毅然进行改革，下列不属于孝文帝改革的内容的是（  ）

 Many students have hobbies, such as reading, painting, growing vegetables and looking after animals. Some hobbies are relaxing and others are active．Hobbies can make you grow as a person, develop your interests and help you learn new skills．21·cn·jy·com

 David Smith is a student，and one of his hobbies is writing．During the summer of 2010，he spent four weeks at a summer camp．Besides the usual activities，such as sailing and climbing , there was a writing class．"The teacher was a writer, and she asked us to talk about our lives and tell interesting stories. Then she envouraged us to  write about our experiences at the camp,”says David.

 Back at school, David wrote a story about the life of a sixteen-year-old boy, and it came out as a book in 2012. Mang young people love his book, and as a result, David has become a successful young writer.

 David has been very lucky because his hobby has brought him pleasure and success. But writing is not his only hobby. He is also interested in many other things. "I like playing volleyball，too，"says David．"I spend some of my free time playing volleyball for my school team．Maybe I'll write about my volleyball team in my future books.”