Consider any arbitrary linear combination of the vectors [tex]\mathbf r,\mathbf u\,\mathbf x[/tex]. We have
[tex]c_1\mathbf r+c_2\mathbf u+c_3\mathbf x=c_1\mathbf r+c_2\mathbf u+c_3(5\mathbf r+\mathbf u+\mathbf d)[/tex]
[tex]=(c_1+5c_3)\mathbf r+(c_2+c_3)\mathbf u+c_3\mathbf d[/tex]
[tex]=c_4\mathbf r+c_5\mathbf u+c_6\mathbf d[/tex]
We know [tex]\mathbf r,\mathbf u,\mathbf d[/tex] are linearly dependent, which means there must exist some choice of not all zero constants [tex]c_4,c_5,c_6[/tex] such that the combination above gives the zero vector. So [tex]T=\{\mathbf r,\mathbf u,\mathbf x\}[/tex] is a set of linearly dependent vectors.
