证明:
假设a1+a2,a2+a3,a3+a4,a4+a1,线性相关
则存在不全为0的k1,k2,k3,k4
st.
k1(a1+a2)+k2(a2+a3)+k3(a3+a4)+k4(a4+a1)=0
(k1+k4)a1+(k2+k1)a2+(k2+k3)a3+(k3+k4)a4=0
k1,k2,k3,k4不全为0
则k1+k2,k2+k3,k3+k4,k4+k1不全为0
所以
a1,a2,a3,a4线性相关,
与题意矛盾
所以
a1+a2,a2+a3,a3+a4,a4+a1,线性无关
证明:
假设a1+a2,a2+a3,a3+a4,a4+a1,线性相关
则存在不全为0的k1,k2,k3,k4
st.
k1(a1+a2)+k2(a2+a3)+k3(a3+a4)+k4(a4+a1)=0
(k1+k4)a1+(k2+k1)a2+(k2+k3)a3+(k3+k4)a4=0
k1,k2,k3,k4不全为0
则k1+k2,k2+k3,k3+k4,k4+k1不全为0
所以
a1,a2,a3,a4线性相关,
与题意矛盾
所以
a1+a2,a2+a3,a3+a4,a4+a1,线性无关