e often write vectors in $R^n$ as rows.

xercise $5\mathrm{.}1\mathrm{.}1$ In each case determine whether $U$ is a ibspace of $R^3.$ Support your answer.

a$.$ and $t$ in $R.$

b$.$ and $t$ in $R.$

c$.,$ and $t$ in $R,-r+3s+2t=0.$

d$.$ and $s$ in $R.$

e$.|=0$ and $s$ in $\{$:$R\}.$