Ta có:
$C = \dfrac{3^{10}+1}{3^9+1}$
$C : 3 = \dfrac{3^{10} + 1}{3^{10} + 3}$
$C : 3 = \dfrac{3^{10} + 3 - 2}{3^{10} + 3}$
$C : 3 =1 - \dfrac{2}{3^{10}+3}$
$D = \dfrac{3^{9}+1}{3^8+1}$
$D: 3 = \dfrac{3^{9} + 1}{3^{9} + 3}$
$D : 3 = \dfrac{3^{9} + 3 - 2}{3^{9} + 3}$
$D : 3 =1 - \dfrac{2}{3^{9}+3}$
Vì : $\dfrac{2}{3^{10} + 3} < \dfrac{2}{3^{9} + 3}$
$⇒ 1 - \dfrac{2}{3^{10}+3} > 1 - \dfrac{2}{3^{9}+3}$
$⇒ C>D$
