Let $z=x+iy$ be a complex number with $x$ and $y$ rational and with $|z| = 1.$

(a) Find two such complex numbers.
(b) Show that $|z^{2n}-1|=2|\sin n\theta|,$ where $z=e^{i\theta}.$
(c) Show that $|z^2n -1|$ is rational for every $n.$
(a) We need to find two complex numbers \( z = x + iy \) where \( x \) and \( y \) are rational and \( |z| = 1 \).

1. Given \( |z| = 1 \), we have:
   \[
   |z| = \sqrt{x^2 + y^2} = 1 \implies x^2 + y^2 = 1
   \]
2. We need \( x \) and \( y \) to be rational numbers. Some rational solutions to this equation are:
   \[
   (x, y) = (1, 0), (0, 1), (-1, 0), (0, -1)
   \]
3. Therefore, the corresponding complex numbers are:
   \[
   z = 1, i, -1, -i
   \]

(b) We need to show that \( |z^{2n} - 1| = 2|\sin n\theta| \) where \( z = e^{i\theta} \).

1. Let \( z = e^{i\theta} = \cos \theta + i \sin \theta \).
2. Then:
   \[
   z^{2n} = (e^{i\theta})^{2n} = e^{i(2n\theta)} = \cos(2n\theta) + i \sin(2n\theta)
   \]
3. We need to find \( |z^{2n} - 1| \):
   \[
   |z^{2n} - 1| = |(\cos(2n\theta) + i \sin(2n\theta)) - 1| = |(\cos(2n\theta) - 1) + i \sin(2n\theta)|
   \]
4. Using the modulus of a complex number:
   \[
   |(\cos(2n\theta) - 1) + i \sin(2n\theta)| = \sqrt{(\cos(2n\theta) - 1)^2 + (\sin(2n\theta))^2}
   \]
5. Simplify the expression inside the square root:
   \[
   (\cos(2n\theta) - 1)^2 + (\sin(2n\theta))^2 = \cos^2(2n\theta) - 2\cos(2n\theta) + 1 + \sin^2(2n\theta)
   \]
6. Using the Pythagorean identity \( \cos^2(2n\theta) + \sin^2(2n\theta) = 1 \):
   \[
   \cos^2(2n\theta) - 2\cos(2n\theta) + 1 + \sin^2(2n\theta) = 1 - 2\cos(2n\theta) + 1 = 2 - 2\cos(2n\theta)
   \]
7. Therefore:
   \[
   |z^{2n} - 1| = \sqrt{2 - 2\cos(2n\theta)}
   \]
8. Using the double-angle identity \( 1 - \cos(2n\theta) = 2\sin^2(n\theta) \):
   \[
   2 - 2\cos(2n\theta) = 4\sin^2(n\theta)
   \]
9. Thus:
   \[
   |z^{2n} - 1| = \sqrt{4\sin^2(n\theta)} = 2|\sin(n\theta)|
   \]

(c) We need to show that \( |z^{2n} - 1| \) is rational for every \( n \).

1. From part (b), we have:
   \[
   |z^{2n} - 1| = 2|\sin(n\theta)|
   \]
2. Since \( z = x + iy \) with \( x \) and \( y \) rational and \( |z| = 1 \), \( \theta \) must be such that \( \cos \theta \) and \( \sin \theta \) are rational.
3. For \( \theta \) such that \( \cos \theta \) and \( \sin \theta \) are rational, \( \sin(n\theta) \) will also be rational for any integer \( n \).
4. Therefore, \( 2|\sin(n\theta)| \) will be rational for any integer \( n \).

The final answer is \( \boxed{ z = 1, i, -1, -i } \) for part (a), \( |z^{2n} - 1| = 2|\sin n\theta| \) for part (b), and \( |z^{2n} - 1| \) is rational for every \( n \) for part (c).