`\text {@Doraemon}`
Ta có: $78^{12}$ - $78^{11}$ = $78^{11}$ . (78-1) = $78^{11}$ . 77
$78^{11}$ - $78^{10}$ = $78^{10}$ . (78-1) = $78^{10}$ . 77
Vì $78^{11}$ > $78^{10}$ nên $78^{11}$ . 77 > $78^{10}$ . 77
⇒$78^{12}$ - $78^{11}$ > $78^{11}$ - $78^{10}$
Vậy $78^{12}$ - $78^{11}$ > $78^{11}$ - $78^{10}$