\hspace{-16}%20$If%20$\vec{u}\times%20\vec{v}$%20be%20a%20Unit-Vector.%20If%20$\vec{w}$%20be%20a%20vector%20such%20that\\\\%20$\vec{w}\times%20u=\vec{v}-\vec{w}$%20and%20$\left|\left(\vec{u}\times%20\vec{v}\right).\vec{w}\right|=\frac{1}{2}$\\\\%20Then%20How%20can%20I%20prove%20$\vec{u}.\vec{v}=0$