This looks like a question on complex numbers. Let (a (2p)th root of unity). Then is also a primitive (2p)th root of unity, and the roots of can be taken to be . Therefore .

Now let and . Then . Factorise the numerator using the result in the previous paragraph with (and ), and the denominator with . This gives

(the products going from k=0 to 2p–1).

But and , so that gives the result.