问题 问答题

设n维向量组α1,α2,…,αs-1线性无关,且
αs1+2α2+3α3+…+(s-1)αs-1
(Ⅰ)证明:线性齐次方程组
α1x12x2+…+αi-1xi-1i+1xi+1+…+αsxs=0 (*)
只有零解.i=1,2,…,s.
(Ⅱ)求线性非齐次方程组
α1x12x2+…+αsxs1+2α2+…+sαs. (**)
的通解.

答案

参考答案:方法一
齐次线性方程组α1x12x2+…+αi-1xi-1i+1xi+1+…+αsxs=0只有零解
[*]r(α1,α2,…,αi-1,αi+1,…,αs)=s-1(未知量个数)
[*]α1,α2,…,αi-1,αi+1,…,αs线性无关.
设有数是k,k2,…,ki-1,ki+1,…,ks,使得
k1α1+k2α2+…+ki-1αi-1+ki+1αi+1+…+ksαs=0 (1)
将题设条件 α21+2α2+…+(s-1)αs-1代入(1),得
k1α1+k2α2+…+ki-1αi-1+ks-1αs-1ks1+2α2+…+(s-1)αs-1=0
即 (k1+ks1+(k2+2ks2+…+ki-1+(i-1)ksαi-1+iksαi+
Lki+1+(i+1)ksαi+1+…+ks-1+(s-1)ksαs-1=0 (2)
由题设,α12,…,αs-1线性无关,
[*]
因i≠0,由iks=0,得ks=0,从而有k1=k2=…=ki-1=ki+1=…=ks-1=0.
得证:α1,α2,…,αi-1i+1,αs线性无关,方程组(1)只有零解.
方法二:
[*]
因α1,α2,…,αs-1线性无关,r(α1,α2,…,αs-1)=s-1,|C|=i(-1)i+s-1≠O,故r(α1,α2,…,αi-1,αi+1,…,αs)=s-1,α1,α2,…,αi-1,αi+l,…,αs,线性无关,故方程组(1)只有零解.
方法三:
证:由向量(Ⅰ)α1,αi-1,αi+1,…αs和α1,α2,…αs(Ⅱ)可以相互表示,得(Ⅰ)线性无关,方程组(Ⅰ)只有零解。
(Ⅱ)方程组(2)的系数矩阵的秩为
r(α1,α2,…,αs)=s-1.
故其通解的结构形式为kξ+η,.
其中对应齐次方程组的基础解系为ξ=1,2,…,s-1,-1T,非齐次的一个特解为
η=1,2,…,sT
故方程组(2)的通解为
k1,2,…,s-1,-1T+1,2,…,sT

阅读理解
给下面每段选择一个合适的主题。
( )1. Mid-Autumn Day is a Chinese traditional festival.  On that night, families eat mooncakes
          together and enjoy the full moon in the open air. Thanksgiving Day is an important festival
           in America.  People get together to eat turkeys and pies to celebrate the harvest. 
( )2.  Travelling by air is fast, but expensive and sometimes dangerous: Travelling by train is quite
        cheap and safe, but a bit slow. Travelling by sea is not very expensive; you can have fresh air and    
         enjoy beautiful scenery (风景) ,  but sometimes it is not very safe and you may feel seasick (晕船).   
( )3.  A man on the motorbike was driving too fast and hit a bag of rice on  the road.  He was badly
             hurt.  Just then,  some children saw it happen and took him to the hospital. 
( )4. You may see the English words "Made in Japan" or "Made in Germany"  written on something. 
           But English isn't the first language in these countries.
( )5. The quickest way to make friends with people is to smile.  When you smile, people think you
             are friendly and easy to talk to.                                                                                      
A. A traffic accident
B. Different ways of travelling
C. Different countries, different festivals
D. Everyone needs friends
E.  English is used as a foreign language in Japan or Germany
F.  How to make friends in the quickest way
简答题