part ii)
The question states that we should use the result from part i. Lets use the statement that n is divisible by 4, that is we suppose that n=4m, m\in\mathbb{Z}.
Then we put that into the result from part i, 2(\sqrt{2})^{4m}\cos(m\pi)=2(\sqrt{2})^{4m}(-1)^{m} convert it back into terms of...