Ý bạn là thế này ư

ab + a + b = 84

10a + b + a + b = 84

11a + 2b = 84 (1)

Vì b là chữ số nên \(2b\le18\)

\(\Rightarrow11a\ge84-18=66\Rightarrow a\ge6\)

Mặt khác, \(11a< 84\Rightarrow a< 8\)

Ta có: \(6\le a< 8\) (1)

Từ (1), ta có: \(11a⋮2\) (vì \(84⋮2,2b⋮2\) )

\(\Rightarrow a⋮2\left(2\right)\)

Từ (1) và (2) tìm được a = 6

Sau đó thay vào tính được b = 9

Ta có : A = a²(a-1)-b²(b-1)+ab-3ab(a-b+1)

                  <=> a³ -a² +b²-b³+ab -3a²b+3ab²-3ab

                  <=> (a³-3a²b+3ab²-b³)+(a²-2ab+b²)

                  <=> (a-b)³+(a-b)²

                 <=> 7³+7² = 392      (Vì a-b=7)

Vậy A=392

nha ^^

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