How would I go about proving the statement: if $D \subseteq \mathbb{F}_7^n$ is a self-dual code, then $2<d(D)<(n+3)/2$.
Where $d(D)$ is the minimum distance of the code D.
I'm not sure where to go from the fact that since $D$ is self dual the code is also self orthogonal and $n=2k$.