Given four non zero vectors $\vec a,\vec b, \vec c$ and $\vec d$. The vectors $\vec a,\vec b$ and $\vec c$ are coplanar but not collinear pair by pair and $\vec d$ is not coplanar with vectors $\vec a,\vec b$ and $\vec c$ and $\widehat{(\vec a\; \vec b)}=\widehat{(\vec b\, \vec c)}=π/3$, $\widehat{(\vec d\;\vec a)}=m$ and $\widehat{(\vec d\; \vec b)}= n$, prove that $\widehat{(\vec d\; \vec c)}=cos^{-1} (cos\,n- cos\,m) $.
I am not getting any start how to do it , can anybody help me in this.