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7 tháng 4 2016

Có:\(\frac{1}{2}<\frac{2}{3}\)

\(\frac{3}{4}<\frac{4}{5}\)

\(\frac{5}{6}<\frac{6}{7}\)

..............

\(\frac{2011}{2012}<\frac{2012}{2013}\)=>A<\(\frac{2}{3}.\frac{4}{5}.....\frac{2012}{2013}\)

=> A2<\(\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.\frac{4}{5}.....\frac{2011}{2012}.\frac{2012}{2013}\)

=> A2<\(\frac{1.2.3.4.....2011.2012}{2.3.4.5.....2012.2013}\)

=>A2<\(\frac{1}{2013}\)=>DPCM

12 tháng 9 2021

a ) 

Theo bài ra: (a - 4) chia hết cho 5 => (a - 4) + 20 chia hết cho 5 => a + 16 chia hết cho 5

(a - 5) chia hết cho 7 => (a - 5) + 21 chia hết cho 7 => a + 16 chia hết cho 7

(a - 6) chia hết cho 11 => (a - 6) + 22 chia hết cho 11 => a + 16 chia hết cho 11 

=> a + 16 thuộc BC(5; 7; 11) 

Mà BCNN(5; 7; 11) = 385

=> a + 16 thuộc B(385) = {0; 385; 770; ...}

=> a thuộc {-16; 369; 754;...}

Vì a là số tự nhiên nhỏ nhất

=> a = 369 

b ) \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}.\)

Ta có : 

\(\frac{1}{2^2}=\frac{1}{2.2}< \frac{1}{1.2}\)

\(\frac{1}{3^2}=\frac{1}{3.3}< \frac{1}{2.3}\)

.....................

\(\frac{1}{2012^2}=\frac{1}{2012.2012}< \frac{1}{2011.2012}\)

Ta có :

\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}< \frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{2011.2012}\)

\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}< 1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{2011}-\frac{1}{2012}\)

\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}< 1-\frac{1}{2012}\)

\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}.< \frac{2011}{2012}\)

Mà \(\frac{2011}{2012}< 1\)

\(\Rightarrow\)\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+.......+\frac{1}{2011^2}+\frac{1}{2012^2}< 1\)

12 tháng 9 2021

\(b)\)\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}\)

\(< \)\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{2010.2011}+\frac{1}{2011.2012}\)

\(< \)\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{2011}-\frac{1}{2012}\)

\(< \)\(1-\frac{1}{2012}\)\(=\frac{2011}{2012}< 1\)

Vậy Biểu thức    \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}\)\(< 1\)

25 tháng 2 2018

Dễ k cho mình trước rồi mình làm cho

25 tháng 2 2018

K phai lop 7 nen k phai lam. Biet dau ma lam 

26 tháng 4 2016

c)\(A=1+\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+....+\frac{1}{2^{2012}}\)

\(2A=2\left(1+\frac{1}{2}+\frac{1}{2^2}+.....+\frac{1}{2^{2012}}\right)\)

\(2A=2+1+\frac{1}{2^2}+\frac{1}{2^3}+.....+\frac{1}{2^{2011}}\)

\(2A-A=\left(2+1+\frac{1}{2^2}+\frac{1}{2^3}+....+\frac{1}{2^{2011}}\right)-\left(1+\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+....\frac{1}{2^{2012}}\right)\)

\(A=2-\frac{1}{2^{2012}}\)

26 tháng 4 2016

1/

A=1/1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100

A=1/1-1/100

Vì 1/100>0

-->1/1-1/100<1

-->A<1

27 tháng 2 2016

Ta có : \(\frac{1}{2^2}<\frac{1}{1.2};\frac{1}{3^2}<\frac{1}{2.3};\frac{1}{4^2}<\frac{1}{3.4};...;\frac{1}{2011^2}<\frac{1}{2010.2011};\frac{1}{2012^2}<\frac{1}{2011.2012}\)

=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}<\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2010+2011}+\frac{1}{2011.2012}\)

=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}<\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{2010}-\frac{1}{2011}+\frac{1}{2011}-\frac{1}{2012}\)

=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}<\frac{1}{1}-\frac{1}{2012}\)

Vì \(\frac{1}{2012}>0\) => \(\frac{1}{1}-\frac{1}{2012}<1\)

=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}<1\)

27 tháng 2 2016

zee that tri tuệ ve toan day so; ok 10đ

5 tháng 7 2018

Ta có:\(\frac{1}{2^2}< \frac{1}{1.2};\frac{1}{3^2}< \frac{1}{2.3};..........;\frac{1}{2012^2}< \frac{1}{2011.2012}\)

Nên \(\frac{1}{2^2}+\frac{1}{3^2}+........+\frac{1}{2012^2}< \frac{1}{1.2}+\frac{1}{2.3}+.......+\frac{1}{2011.2012}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+.........+\frac{1}{2011}-\frac{1}{2012}\)

\(=1-\frac{1}{2012}< 1\)

5 tháng 7 2018

ta có: \(\frac{1}{2^2}< \frac{1}{1.2};\frac{1}{3^2}< \frac{1}{2.3};\frac{1}{4^2}< \frac{1}{3.4};\frac{1}{5^2}< \frac{1}{4.5};...;\frac{1}{2011^2}< \frac{1}{2010.2011};\)\(\frac{1}{2012^2}< \frac{1}{2011.2012}\)

\(\Rightarrow\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}\)\(< \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{2010.2011}+\frac{1}{2011.2012}\)\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{2010}-\frac{1}{2011}+\frac{1}{2011}-\frac{1}{2012}\)

\(=1-\frac{1}{2012}< 1\)

\(\Rightarrow\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+...+\frac{1}{2011^2}+\frac{1}{2012^2}< 1\left(đpcm\right)\)

31 tháng 3 2019

Ta có

\(\frac{1}{2^2}< \frac{1}{1.2};\frac{1}{3^2}< \frac{1}{2.3};.....;\frac{1}{2012^2}< \frac{1}{2011.2012}\)

=> \(\frac{1}{2^2}+\frac{1}{3^2}+...+\frac{1}{2012^2}< \frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{2011.2012}\)

= 1-\(\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+....+\frac{1}{2011}-\frac{1}{2012}\)

=1-\(\frac{1}{2012}\)=\(\frac{2011}{2012}< 1\)

Vậy \(\frac{1}{2^2}+\frac{1}{3^2}+....+\frac{1}{2012^2}< 1\)