if $A,B,C,D$ be $4$ points in a space satisfying $|\vec{AB}| = 3\;,|\vec{BC}| = 7\;|\vec{CD}| = 11\;|\vec{DA}| = 9$.
find $\vec{AD}\cdot \vec{BC}=$
using $|\vec{DA}|^2+|\vec{BC}|^2=|\vec{AB}|^2+|\vec{CD}|^2$
i could not go further, could some help me with this, thanks