tính tổng
a= \(\frac{1}{1.5}+\frac{1}{1.6}+\frac{1}{1.7}+....+\frac{1}{1.100}\)
K
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N
20 tháng 6 2017
\(M=\frac{16}{1.5}+\frac{16}{5.9}+........+\frac{16}{2017.2021}\)
\(M=4.\left(\frac{4}{1.5}+\frac{4}{5.9}+.......+\frac{4}{2017.2021}\right)\)
\(M=4.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+.........+\frac{1}{2017}-\frac{1}{2021}\right)\)
\(M=4.\left(1-\frac{1}{2021}\right)\)
\(M=4.\frac{2020}{2021}\)
\(M=\frac{8080}{2021}\)
\(N=\frac{1}{1.7}+\frac{1}{7.13}+.......+\frac{1}{2007.2013}\)
\(N=\frac{1}{6}.\left(\frac{6}{1.7}+\frac{6}{7.13}+........+\frac{6}{2007.2013}\right)\)
\(N=\frac{1}{6}.\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+......+\frac{1}{2007}-\frac{1}{2013}\right)\)
\(N=\frac{1}{6}.\left(1-\frac{1}{2013}\right)\)
\(N=\frac{1}{6}.\frac{2012}{2013}\)
\(N=\frac{1006}{6039}\)
20 tháng 6 2017
\(N=\frac{1}{1.7}+\frac{1}{7.13}+...+\frac{1}{2007.2013}\)
\(N=\frac{1}{1}-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+...+\frac{1}{2007}-\frac{1}{2013}\)
\(N=1-\frac{1}{2013}\)
\(N=\frac{2012}{2013}\)
20 tháng 4 2020
Sửa đề:
\(E=\frac{1}{1.101}+\frac{1}{2.102}+\frac{1}{3.103}+...+\frac{1}{10.110}\)
=> \(100.E=\frac{100}{1.101}+\frac{100}{2.102}+\frac{100}{3.103}+...+\frac{100}{10.110}\)
\(=\frac{101-1}{1.101}+\frac{102-2}{2.102}+\frac{103-3}{3.103}+...+\frac{110-10}{10.110}\)
\(=1-\frac{1}{101}+\frac{1}{2}-\frac{1}{102}+\frac{1}{3}-\frac{1}{103}+...+\frac{1}{10}-\frac{1}{110}\)
\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{10}\right)-\left(\frac{1}{101}+\frac{1}{102}+\frac{1}{103}+...+\frac{1}{110}\right)\)
\(F=\frac{1}{1.11}+\frac{1}{2.12}+\frac{1}{3.13}+...+\frac{1}{100.110}\)
=> \(10F=\frac{10}{1.11}+\frac{10}{2.12}+\frac{10}{3.13}+...+\frac{10}{100.110}\)
\(=\frac{11-1}{1.11}+\frac{12-2}{2.12}+\frac{13-3}{3.13}+...+\frac{110-100}{100.110}\)
\(=1-\frac{1}{11}+\frac{1}{2}-\frac{1}{12}+\frac{1}{3}-\frac{1}{13}+...+\frac{1}{100}-\frac{1}{110}\)
\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{10}\right)+\left(-\frac{1}{11}+\frac{1}{11}\right)+\left(-\frac{1}{12}+\frac{1}{12}\right)+...+\left(-\frac{1}{100}+\frac{1}{100}\right)\)
\(-\left(\frac{1}{101}+\frac{1}{102}+...+\frac{1}{110}\right)\)
\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{10}\right)-\left(\frac{1}{101}+\frac{1}{102}+...+\frac{1}{110}\right)=100E\)
=> 10 F = 100 E
=> \(\frac{E}{F}=\frac{10}{100}=\frac{1}{10}\)
BM
3
20 tháng 4 2018
\(A=\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+....+\frac{1}{496}-\frac{1}{501}\right):5\)
\(A=\left(1-\frac{1}{501}\right):5\)
\(A=\frac{500}{501}:5=\frac{100}{501}\)
20 tháng 4 2018
Ta có : \(A=\frac{1}{1.6}+\frac{1}{6.11}+\frac{1}{11.16}+...+\frac{1}{496.501}\)
\(\Rightarrow\) \(A=\frac{1}{5}\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{496}-\frac{1}{501}\right) \)
\(\Rightarrow\) \(A=\frac{1}{5}\left(1-\frac{1}{501}\right)\)
\(\Rightarrow\) \(A=\frac{1}{5}.\frac{501-1}{501}=\frac{1}{5}.\frac{500}{501}\)
\(\Rightarrow\) \(A=\frac{1.500}{5.501}=\frac{20}{1.501}=\frac{20}{501}\)
Vậy \(A=\frac{20}{501}\)
TA
27 tháng 3 2017
- a) giai ta co (1/2.5=1/2-1/5)+(1/5.8=1/5-1/8)+....+(1/2009.2012=1/2009-1/2012) =>a=1/2-1/5+1/5-1/8+...+1/2009-1/2012 <=>1/2-1/2012 =>a=1005/2012 câu b bằng nhau nhhung minh không th
- e giải ra được
HT
15 tháng 8 2015
A = \(\frac{1}{2}+\frac{1}{2}.\frac{1}{3}+\frac{1}{3}.\frac{1}{4}+...+\frac{1}{9}.\frac{1}{10}\)
A = \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)
A = \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
A = \(1-\frac{1}{10}\)
A = \(\frac{9}{10}\)
15 tháng 8 2015
1/2=1-1/2 ; 1/2.1/3=1/2-1/3 ; 1/3.1/4=1/3-1/4...v...v
Vậy A bằng: 1-1/2+1/2-1/3+1/3-1/4+1/4-1/5.............+1/8-1/9+1/9-1/10
=1-1/10=9/10
10 tháng 4 2016
a, \(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{24.25}\)
\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{24}-\frac{1}{25}\)
\(=\frac{1}{5}-\frac{1}{25}\)
\(=\frac{4}{25}\)
b, \(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{97.99}\)
Gọi biểu thức trên là A