Giải thích các bước giải:
a.Ta có :
$A=3^{n+2}-2^{n+2}+3^n-2^n=3^n.3^2-2^n.2^2+3^n-2^n=9.3^n+3^n-4.2^n-2^n=10.3^n-5.2^n$
Vì $n\in N*\to 2^n\quad\vdots\quad 2\to 5.2^n\quad\vdots\quad 10$
$\to 10.3^n-5.2^n\quad\vdots\quad 10$
$\to 3^{n+2}-2^{n+2}+3^n-2^n\quad\vdots\quad 10$