Consider the vectors

vec$(a)=3$vec$(i)$ vec$(j)-$vec$(k),$vec$(b)=-$vec$(i)3$vec$(j)0$vec$(k),$vec$(c)=$vec$(i)0\mathrm{.}333333$vec$(j)0\mathrm{.}333333$vec$(k)$

vec$(d)=$vec$(i)$ vec$(j)4$vec$(k),$vec$(g)=-3$vec$(i)-$vec$(j)$ vec$(k).$

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

$($a$)$ Are perpendicular?

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$($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?

$($Enter none or a pair or list of pairs, e$.,,$ 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?

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$($Enter none or a pair or list of pairs, e$.,$ if 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 $(a,b),(a,c).)$