(完整word版)高等代数(北大版)2011-2012第一学期考试卷A答案.pdf
高 等 代 数(北 大 版)2011-2012第 一 学 期 考 试 卷 答 案(A 卷)考 试(考 查):考 试时 间:2007年 1 月日本 试 卷 共 6 页,满 分 100 分;考 试 时 间:120 分 钟题 号一二三四总 分阅 卷 教 师 签 名12312得 分一、选择题(本大题共8 个小题,每空4 分,共 32 分.请在每小题的四个备选答案中选出一个正确的答案,并将其号码填入题后的括号内).1、若n阶矩阵 A 与 B 相似,则【A】.A 与 B 有相同特征值B.A 与 B 有不同特征值C.A 与 B 有相同特征向量D.A 与 B 有不同特征向量2、下列向量组中,线性无关的是【D】A 0B,0C1221,rm其中D,21r,其中任一向量都不能表成其余向量的线性组合.3、已知112212112212(,),(,)(,),(,)aab bccdd与是向量空间2F的两个基,则从基12,到基12,的过渡矩阵为【A】A111112222abcdabcdB111112222cdabcdabC112121212aaccbbddD112121212ccaaddbb4、下列子集中,作成向量空间Rn的子空间的是【B】A121(,)|1 nniia aaaB0|),(121niinaaaaC12(,)|,1,2,nia aaaZ inD 1|),(121niinaaaa5、欧氏空间V 的线性变换是对称变换的充要条件是V,,都有【C】得分评卷人A,)(),(B|)(|C),()(,D把 V 的规范正交基变成V 的规范正交基6、设矩阵A 为 n 阶方阵且|A|=0,则【C】AA 中必有两行或两列的元素对应成比例BA 中至少有一行或一列的元素全为零;CA 中必有一行或一列向量是其余各行或各列向量的线性组合;DA 中任意一行或一列向量是其余各行或列向量的线性组合7、设V是n维向量空间,)(VL的维数为【B】A.nB.2nC.)1(21nnD.无限维8设123,是欧氏空间V 的规范正交基,V且1,1,2,0,3,3,则【A】A.133B.233C.1233D.123二、填空题(本大题共5个小题,每空4 分,共 20 分.请将正确结果填在题中横线上).1、三阶方阵A的特征多项式为32()223Af,则|A-3 .2、设1322A,则向量11是 A的属于特征根 4 的特征向量3、BA,为n阶正交矩阵,且,0|A0|B,则|AB-14、取3/2时,向量组12(1,0,1),(4,3),3(1,3,1)线性相关.得分评卷人文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U75、若 A 是正交矩阵,kR,要使 kA 为正交矩阵,则k=1.三、计算题(本大题共3 个小题,共28 分.请写出必要的推演步骤和文字说明).1、(本小题6 分)在向量空间3R中,求由向量组123(2,3,1),(1,4,2),(5,2,4)所生成子空间的基和维数;解:(解法不唯一)令123215342124A.则只对 A 施行行初变换即可.2 分12410201010011033000A.4 分故12,为所求的一个基,生成子空间的维数是2.6 分2、(本小题8 分)设F x表示数域F 上次数小于3 的多项式连同零多项式构成的向量空间,定义映射:()()f xfx1)验证是线性变换;2)求线性变换在基21,1,12xxx下的矩阵.解:1)(),(),f xg xF xa bF有()()()()()()()()afxbg xaf xbg xafxbgxaf xbg x故:()()f xfx是 F x上的线性变换(也可用线性变换定义验证)3 分2)因为2222(1)00 10(1)0(1)2(1)11 10(1)0(1)2(1)10 1 1(1)0(1)22xxxxxxxxxxxxx关于基21,1,12xxx的矩阵为:得分评卷人4 分5 分6 分文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U70100010008 分3、(本小题14 分)对实对称矩阵求一个正交矩阵U,使UAU 为对角形矩阵.解:2122()212(5)(1)221AxfxxIAxxxx所以特征根为,(二重)分当1时,对应齐次线性方程组为123222022202220 xxx其基础解系为12(1,1,0),(1,0,1)分正交化得1211112(,0),(,)22666分当5时,对应齐次线性方程组为123422024202240 xxx其基础解系为3(1,1,1),化为为单位向量为3111(,)33311 分所以正交矩阵123111263111,26321063U13 分122212221A文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7且使得115U AU14 分四、证明题(本大题共2 个小题,每小题10 分,共 20 分,须写出必要的推理过程和文字说明)1设,21n都是一个欧氏空间的向量,且是n,21的线性组合.证明:如果与i正交,ni,2,1,那么0.证明:令1niiia,则3 分11,0nniiiiiiaa每式 2 分,计 9 分所以 =0 10 分2设123,是三维欧氏空间3R的一个标准正交基,证明:112321233123111(22),(22),(22)333也是3R的一个标准正交基证明:(证明方法不唯一)由123,到123,的过渡矩阵为221333212333122333U4 分U UUUI8 分123,也是标准正交基.10 分得分评卷人文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7文档编码:CH10F6D3Q4G10 HQ7H4H7W2D3 ZA10E10S6M3U7