Cho day $(U_n)$ biet :
                                $\left\{ \begin{array}{l} U_1=1;U_2=2\\ U_{n+2}=9U_{n+1}-18U_n,\forall n\in N^*\end{array} \right.$
         Xac dinh $U_n$
Thay $n=0$ vào công thức truy hồi $\Rightarrow U_0=\frac{7}{18}$
Viết lại công thức truy hồi $U_{n+1}-9U_n+18U_{n-1}=0$
Xét pt: $x^2-9x+18=0$ có 2 nghiệm $x_1=3;x_2=6$
$\Rightarrow U_n=k.x_1^n+l.x_2^n$
Thay $n=0;1$ vào $\Rightarrow \begin{cases}k+l=U_0 \\ 3k+6l=U_1 \end{cases}\Rightarrow k=\frac{4}{9};l=\frac{-1}{18}$
Vậy $U_n=\frac{4}{9}.3^n-\frac{1}{18}.6^n$
ok ok ok. hahaha. cho han xuong di. cho han fly lun :)) –  ♥♥♥ Panda Sơkiu Panda Mập ♥♥♥ 17-02-14 09:52 PM
chep chep, lan` nay` han' k oanh' moi' la` la. a', kkkk hay de ck vote down them vai` lan` nua~ vk nhi? hjhj^^ –  gio_lang_thang 17-02-14 09:51 PM
vk vote down cho han mat 2 diem.hahaha –  ♥♥♥ Panda Sơkiu Panda Mập ♥♥♥ 17-02-14 09:48 PM
uk vk, piet sao duok, han' ta thik V voi vote thui a', con` cam' on hay k cung k quan tam ak :P ng dau ma` la lung` thiet,kkkk –  gio_lang_thang 17-02-14 09:46 PM
ck vote ui. tks han cung hem tac dung ak.hahaha vk vote down day :)) –  ♥♥♥ Panda Sơkiu Panda Mập ♥♥♥ 17-02-14 09:43 PM
cam' on mon nhiu`:)) –  gio_lang_thang 17-02-14 09:21 PM

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