Consider the vectors

vec$(a)=2$vec$(i)+$vec$(j)-$vec$(k),$vec$(b)=-$vec$(i)-2$vec$(j)+$vec$(k),$vec$(c)=-2$vec$(i)-$vec$(j)+$vec$(k)$

vec$(d)=-$vec$(i)-2$vec$(j)+$vec$(k),$vec$(g)=$vec$(i)+$vec$(j)+3$vec$(k).$

Which pairs $($if any$)$ of these vectors are

$($a$)$ Are perpendicular?

$($Enter none or a pair or list of pairs, e$.$g$.,$ if vec$(a)$ is perpendicular to vec$(b)$ and vec$(c),$ enter $(a,b),(a,c).)$

$($b$)$ Are parallel?

none

$($Enter none or a pair or list of pairs, e$.$g$.,$ if vec$(a)$ is parallel to vec$(b)$ and vec$(c),$ enter $(a,b),(a,c).)$

$($c$)$ Have an angles less than $\frac{}{2}$ between them?

$(b,g),(c,g)$

$($Enter none or a pair or list of pairs, e$.$g$.,$ if vec$(a)$ is at an angle less than pi$/2$ from vec$(b)$ and vec$(c),$ enter $(a,b),(a,c).)$

$($d$)$ Have an angle of more than $\frac{}{2}$ between them?

$($Enter none or a pair or list of pairs, e$.$g$.,$ if vec$(a)$ is at an angle greater than pi$/2$ from vec$(b)$ and vec$(c),$ enter $(a,b),(a,c).)$