a, [tex]-\frac{1}{7}S=(-\frac{1}{7})^1+(-\frac{1}{7})^2+...+(-\frac{1}{7})^{2008}[/tex]
[tex]\Rightarrow -\frac{1}{7}S-S=(-\frac{1}{7})^{2008}-(-\frac{1}{7})^0 \Rightarrow -\frac{8}{7}S=(-\frac{1}{7})^{2008}-1 \Rightarrow S=(-\frac{1}{7})^{2007}.\frac{1}{8}+\frac{7}{8}[/tex]
c,[tex]3^{n+2}-2^{n+2}+3^n-2^n=9.3^n-8.2^{n-1}+3^n-2.2^{n-1}=10.3^n-10.2^{n-1}\vdots 10[/tex]
Có gì thắc mắc thì em hỏi lại nhé!