Mathematics

Problem

- If

\mathbf{a} \times \mathbf{b} = \mathbf{c} \times \mathbf{d}

and

\mathbf{a} \times \mathbf{c} = \mathbf{b} \times \mathbf{d}

, then show that

\mathbf{a} - \mathbf{d}

is parallel to

\mathbf{b} - \mathbf{c}