28. Show that a prime divisor $p$ of the Fermat number $F_{n}=2^{2+}+1$ must be of the form $2^{n+2} k+1$. (Hint: Show that $\operatorname{ord}_{p} 2=2^{n+1}$. Then show that $2^{(p-1) / 2} \equiv 1(\bmod p)$ using Theorem 9.4. Conclude that $\left.2^{n+1} \mid(p-1) / 2.\right)$