1/3+2/3bình+3/3lập+........+n/3 mũ n <3/4
giải hộ cái nha cả nhà
[TEX]S=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+ \frac{n-1}{3^{n-1}}+ \frac{n}{3^n}[/TEX]
[TEX]\Rightarrow 3s=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{n}{3^{n-1}}[/TEX]
[TEX]\Rightarrow 2S=3S-S=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{n-1}} -\frac{n}{3^n}<1+\frac{1}{3}+\frac{1}{3^2}+...+ \frac{1}{3^{n-1}}[/TEX]
[TEX]\Rightarrow 2S(1-\frac{1}{3}) < (1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{n-1}})(1-\frac{1}{3})=1-(\frac{1}{3})^n <1[/TEX] (hằng đẳng thức [tex]a^n-b^n[/tex]
[TEX]\Rightarrow \frac{4}{3}S<1 \Rightarrow dpcm[/TEX]