a)
$2^{12}$ + $1$ = $(2^{3})^{4}$ + $1^{3}$ = $(2^{4}+1)(2^{2} - 2.1 + 1^{2})$
= (16 + 1)($2^{4}$ - 2.1 + 1²) = 17($2^{4}$ - 2.1 + 1²) chia hết 17
b)
$3^{9}$ - 8 = $(3^{3})^{3}$ - $2^{3}$ = ($3^{3} - 2)(3^{2} + 3^{3}.2 + 2^{2})$ = $25(3^{2} + 3^{3}.2 + 2^{2})$ chia hết 25