Prove that if $\langle\mathbf{v}, \mathbf{w}\rangle$ and $\langle\langle\mathbf{v}, \mathbf{w}\rangle\rangle$ are two inner products on the same vector space $V$, then their sum $\langle\langle\langle\mathbf{v}, \mathbf{w}\rangle\rangle\rangle=\langle\mathbf{v}, \mathbf{w}\rangle+\langle\langle\mathbf{v}, \mathbf{w}\rangle\rangle$ defines an inner product on $V$.