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\(\dfrac{1}{2^2}< \dfrac{1}{1\cdot2};\dfrac{1}{3^2}< \dfrac{1}{2\cdot3};...;\dfrac{1}{100^2}< \dfrac{1}{99\cdot100}\\ \Rightarrow\dfrac{1}{2^2}+\dfrac{1}{3^2}+\dfrac{1}{4^2}+...+\dfrac{1}{100^2}< \dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{99\cdot100}\\ =1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}=1-\dfrac{1}{100}=\dfrac{99}{100}< 1\\ \RightarrowĐpcm\)
Ta có :
\(\dfrac{1}{5^2}< \dfrac{1}{4.5}\)
\(\dfrac{1}{6^2}< \dfrac{1}{5.6}\)
..............
\(\dfrac{1}{100^2}< \dfrac{1}{99.100}\)
\(\Leftrightarrow\dfrac{1}{5^2}+\dfrac{1}{6^2}+.....+\dfrac{1}{100^2}< \dfrac{1}{4.5}+\dfrac{1}{5.6}+....+\dfrac{1}{99.100}=\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+....+\dfrac{1}{99}-\dfrac{1}{100}=\dfrac{1}{4}-\dfrac{1}{100}< \dfrac{1}{4}\left(1\right)\)
Lại có :
\(\dfrac{1}{5^2}>\dfrac{1}{5.6}\)
\(\dfrac{1}{6^2}>\dfrac{1}{6.7}\)
..............
\(\dfrac{1}{100^2}>\dfrac{1}{100.101}\)
\(\Leftrightarrow\dfrac{1}{5^2}+\dfrac{1}{6^2}+......+\dfrac{1}{100^2}>\dfrac{1}{5.6}+\dfrac{1}{6.7}+.....+\dfrac{1}{100.101}=\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+....+\dfrac{1}{100}-\dfrac{1}{101}=\dfrac{1}{5}-\dfrac{1}{101}>\dfrac{1}{6}\left(2\right)\)
Từ \(\left(1\right)+\left(2\right)\Leftrightarrow\dfrac{1}{6}< \dfrac{1}{5^2}+\dfrac{1}{6^2}+....+\dfrac{1}{100^2}< \dfrac{1}{4}\)
\("!"\) là giai thừa đó bạn ạ .
\(VD:\) \(3!=1.2.3=6\)
\(4!=1.2.3.4=24\)
TA CÓ 1/2^2=1/4
1/3^2<1/2.3=1/2-1/3
1/4^2<1/3.4=1/3-1/4
1/100^2<1/99.100
=>1/2^2+2/3^2+.....+1/100^2<1/1.2+1/2.3+..+1/99.100
=1-99/100=99/100<1
Đặt \(A=\dfrac{1}{2}+\dfrac{1}{2^2}+...+\dfrac{1}{2^{100}}\)
\(\Rightarrow2A=1+\dfrac{1}{2}+...+\dfrac{1}{2^{99}}\)
\(\Rightarrow2A-A=1-\dfrac{1}{2^{100}}\)
\(\Rightarrow A=1-\dfrac{1}{2^{100}}< 1\) (đpcm)
\(A=\dfrac{1}{2}+\dfrac{1}{2^2}+...+\dfrac{1}{2^{100}}\)
\(2A=1+\dfrac{1}{2}+...+\dfrac{1}{2^{99}}\)
\(2A-A=1-\dfrac{1}{2^{100}}\)
\(A=1-\dfrac{1}{2^{100}}< 1\left(đpcm\right)\)