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Ta có:
\(A=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}+\frac{101}{3^{101}}\)
\(\Rightarrow3\cdot A=3\cdot\left(\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}+\frac{101}{3^{101}}\right)\)
\(\Rightarrow3\cdot A=3\cdot\frac{1}{3}+3\cdot\frac{2}{3^2}+3\cdot\frac{3}{3^3}+...+3\cdot\frac{100}{3^{100}}+3\cdot\frac{101}{3^{101}}\)
\(\Rightarrow3\cdot A=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}+\frac{101}{3^{100}}\)
\(\Rightarrow3\cdot A-A=\left(1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}+\frac{101}{3^{100}}\right)-\left(\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}+\frac{101}{3^{101}}\right)\)
\(\Rightarrow2\cdot A=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}+\frac{101}{3^{100}}-\frac{1}{3}-\frac{2}{3^2}-\frac{3}{3^3}-...-\frac{100}{3^{100}}-\frac{101}{3^{101}}\)
\(\Rightarrow2\cdot A=1+\left(\frac{2}{3}-\frac{1}{3}\right)+\left(\frac{3}{3^2}-\frac{2}{3^2}\right)+...+\left(\frac{101}{3^{100}}-\frac{100}{3^{100}}\right)-\frac{101}{3^{101}}\)
\(\Rightarrow2\cdot A=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}-\frac{101}{3^{101}}\)
Khi đặt \(S=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}\) thì ta sẽ có 2 điều:
- Điều 1: Khi đó:
\(2\cdot A=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}-\frac{101}{3^{101}}\)
\(\Rightarrow2\cdot A=S-\frac{101}{3^{101}}\)
\(\Rightarrow2\cdot A< S\) ( 1 )
Điều 2: Khi đó:
\(S=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}\)
\(\Rightarrow3\cdot S=3\cdot\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}\right)\)
\(\Rightarrow3\cdot S=3\cdot1+3\cdot\frac{1}{3}+3\cdot\frac{1}{3^2}+...+3\cdot\frac{1}{3^{100}}\)
\(\Rightarrow3\cdot S=3+1+\frac{1}{3}+...+\frac{1}{3^{99}}\)
\(\Rightarrow3\cdot S-S=\left(3+1+\frac{1}{3}+...+\frac{1}{3^{99}}\right)-\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}\right)\)
\(\Rightarrow2\cdot S=3+1+\frac{1}{3}+...+\frac{1}{3^{99}}-1-\frac{1}{3}-\frac{1}{3^2}-...-\frac{1}{3^{100}}\)
\(\Rightarrow2\cdot S=3+\left(1-1\right)+\left(\frac{1}{3}-\frac{1}{3}\right)+\left(\frac{1}{3^2}-\frac{1}{3^2}\right)+...+\left(\frac{1}{3^{99}}-\frac{1}{3^{99}}\right)-\frac{1}{3^{100}}\)
\(\Rightarrow2\cdot S=3+0+0+0+...+0-\frac{1}{3^{100}}\)
\(\Rightarrow2\cdot S=3-\frac{1}{3^{100}}\)
Do \(3-\frac{1}{3^{100}}< 3\) nên:
\(\Rightarrow2\cdot S< 3\)
\(\Rightarrow S< \frac{3}{2}\) ( 2 )
Từ ( 1 ) và ( 2 ), theo tính chất bắc cầu suy ra:
\(2\cdot A< \frac{3}{2}\)
\(\Rightarrow A< \frac{3}{2}:2\)
\(\Rightarrow A< \frac{3}{2\cdot2}\)
\(\Rightarrow A< \frac{3}{4}\) ( đpcm )
Ta có :
\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{100^2}< \frac{1}{2^2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)
\(\Rightarrow A< \frac{1}{2^2}+\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\right)\)
\(\Rightarrow A< \frac{1}{4}+\left(\frac{1}{2}-\frac{1}{100}\right)\)
\(\Rightarrow A< \left(\frac{1}{4}+\frac{1}{2}\right)-\frac{1}{100}\)
\(\Rightarrow A< \frac{3}{4}-\frac{1}{100}\)
\(\Rightarrow A< \frac{3}{4}\left(Đpcm\right)\)
~ Ủng hộ nhé
1/2!+1/3!+...+1/100!<1/1x2+1/2x3+...+1/99x100
Ta có 1/1x2=1-1/2
1/2x3=1/2-1/3
......
1/99x100=1/99-1/100
=1-1/2+1/2-1/3+...+1/99-1/100
=1-1/100<1
tk mình đi mình nhanh nhất
ta có 1/2!=1/2
1/3!=1/2*3
1/4!<1/3*4
................
1/100!<1/99*100
=>A<1/2+1/2*3+1/3*4+...+1/99*100
=1/1-1/2+1/2-1/3+1/3-1/4+.....+1/99-1/100
=1/1-1/100
=99/100<1
=>A<1