Ta có:
$C = \dfrac{15^{12} + 1}{15^{13} + 1}$
$⇔ 15C = \dfrac{15^{13} + 15}{15^{13} + 1}$
$⇔ 15C = \dfrac{15^{13} + 1 + 14}{15^{13} + 1}$
$⇔ 15C = 1 + \dfrac{14}{15^{13}+1}$
$D = \dfrac{15^{11} + 1}{15^{12} + 1}$
$⇔ 15D = \dfrac{15^{12} + 15}{15^{12} + 1}$
$⇔ 15D = \dfrac{15^{12} + 1 + 14}{15^{12} + 1}$
$⇔ 15D = 1 + \dfrac{14}{15^{12}+1}$
Vì : $\dfrac{14}{15^{13} + 1} < \dfrac{14}{15^{12} + 1}$
$⇒ C < D$
