设有k1,k2,k3,k4使k1(a1+a2)+k2(a2+a3)+k3(a3+a4)+k4(a4+a1)=0
即(k1+k4)a1+(k1+k2)a2+(k2+k3)a3+(k3+k4)a4=0
由题意a1,a2,a3,a4线性无关,则
k1+k4=0
k1+k2=0
k2+k3=0
k3+k4=0
显然k1=k3=1,k2=k4=-1是其一组解,k1,k2,k3,k4都不为0,所以
a1+a2,a2+a3,a3+a4,a4+a1线性相关
设有k1,k2,k3,k4使k1(a1+a2)+k2(a2+a3)+k3(a3+a4)+k4(a4+a1)=0
即(k1+k4)a1+(k1+k2)a2+(k2+k3)a3+(k3+k4)a4=0
由题意a1,a2,a3,a4线性无关,则
k1+k4=0
k1+k2=0
k2+k3=0
k3+k4=0
显然k1=k3=1,k2=k4=-1是其一组解,k1,k2,k3,k4都不为0,所以
a1+a2,a2+a3,a3+a4,a4+a1线性相关