D=(1+1/1.3)(1+1/2.4)....(1+1/17.19)
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HQ
10
23 tháng 3 2015
Ta có:
\(\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)\left(1+\frac{1}{3.5}\right)...\left(1+\frac{1}{17.19}\right)\left(1+\frac{1}{18.20}\right)\)
\(=\frac{4}{1.3}.\frac{9}{2.4}.\frac{16}{3.5}...\frac{324}{17.19}\frac{361}{18.20}\)
\(=\frac{2.2.3.3.4.4...18.18.19.19}{1.3.2.4.3.5...17.19.18.20}\)
Thấy kể từ phân số thứ 2 trở đi đến phân số thứ 2 từ cuối lên, ở tử và mẫu có thừa số a.a thì ở phân số trước và sau phân số đó cũng có mẫu chứa thừa số a nên ta rút gọn chúng.
=2.2.3.3.4.4...18.18.19.19/1.3.2.4.3.5...17.19.18.20
\(=\frac{2}{1.20}\)
\(=\frac{1}{10}\)
O
2
11 tháng 4 2018
\(\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)...\left(1+\frac{1}{99.101}\right)\)
\(=\frac{4}{1.3}.\frac{9}{2.4}....\frac{10000}{99.101}\)
\(=\frac{2.2.3.3...100.100}{1.3.2.4...99.101}\)
\(=\frac{\left(2.3.4...100\right)\left(2.3.4...100\right)}{\left(1.2...99\right)\left(3.4.5...101\right)}\)
\(=\frac{100.2}{101}=\frac{200}{101}\)
YN
12 tháng 4 2021
\(D=\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)\left(1+\frac{1}{3.5}\right)...\left(1+\frac{1}{99.101}\right)\)
\(D=\frac{4}{1.3}.\frac{9}{2.4}.\frac{16}{3.5}...\frac{10000}{99.101}\)
\(D=\frac{2^2}{1.3}.\frac{3^2}{2.4}.\frac{4^2}{3.5}...\frac{100^2}{99.101}\)
\(D=\frac{2.3.4...100}{1.2.3...99}.\frac{2.3.4...100}{3.4.5...101}=100.\frac{2}{101}=\frac{200}{101}\)
Vậy \(D=\frac{200}{101}\)
NT
0
TQ
1
18 tháng 3 2023
\(P=\dfrac{1}{1.3}+\dfrac{1}{2.4}+\dfrac{1}{3.5}+\dfrac{1}{4.6}+...+\dfrac{1}{2021.2023}\)
Ta sẽ "tách" P làm 2 phần:
\(A=\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{2021.2023}\)
\(B=\dfrac{1}{2.4}+\dfrac{1}{4.6}+\dfrac{1}{6.8}+...+\dfrac{1}{2020.2022}\)
Do đó \(P=A+B\)
Ta có \(A=\dfrac{1}{2}\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{2021.2023}\right)\)
\(A=\dfrac{1}{2}\left(\dfrac{3-1}{1.3}+\dfrac{5-3}{3.5}+\dfrac{7-5}{5.7}+...+\dfrac{2023-2021}{2021.2023}\right)\)
\(A=\dfrac{1}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{2021}-\dfrac{1}{2023}\right)\)
\(A=\dfrac{1}{2}\left(1-\dfrac{1}{2023}\right)\)
\(A=\dfrac{1011}{2023}\)
Mặt khác, \(B=\dfrac{1}{2.4}+\dfrac{1}{4.6}+\dfrac{1}{6.8}+...+\dfrac{1}{2020.2022}\)
\(B=\dfrac{1}{2}\left(\dfrac{2}{2.4}+\dfrac{2}{4.6}+\dfrac{2}{6.8}+...+\dfrac{2}{2020.2022}\right)\)
\(B=\dfrac{1}{2}\left(\dfrac{4-2}{2.4}+\dfrac{6-4}{4.6}+\dfrac{8-6}{6.8}+...+\dfrac{2022-2020}{2020.2022}\right)\)
\(B=\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{2020}-\dfrac{1}{2022}\right)\)
\(B=\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{2022}\right)\)
\(B=\dfrac{505}{2022}\)
Từ đó \(P=A+B=\dfrac{1011}{2023}+\dfrac{505}{2022}=\dfrac{3065857}{4090506}\)
NH
1
NN
1
11 tháng 5 2016
\(\left(1+\frac{1}{1.3}\right).\left(1+\frac{1}{2.4}\right).\left(\frac{1}{3.5.}\right).....\left(1+\frac{1}{99.101}\right)\)
\(=\frac{4}{3}.\frac{9}{8}.\frac{16}{15}.....\frac{10000}{9999}\)
\(=\frac{2^2}{1.3}.\frac{3^2}{2.4}.\frac{4^2}{3.5}.....\frac{100^2}{99.101}\)
\(=\frac{2^2.3^2.4^2.5^2.....98^2.99^2.100^2}{1.2.3^2.4^2.5^2......99^2.100.101}\)
\(=\frac{2.100}{1.101}\)
\(=\frac{200}{101}\)
NL
1
20 tháng 4 2018
Ta có :\(\left(1+\frac{1}{1.3}\right)+\left(1+\frac{1}{2.4}\right)+...+\left(1+\frac{1}{18.20}\right)\)
= \(\frac{4}{1.3}+\frac{9}{2.4}+...+\frac{361}{18.20}\)
= \(\frac{2.2.3.3.4.4.....18.18.19.19}{1.3.2.4.3.5.....17.19.18.20}\)
= \(\frac{2.19}{1.20}=\frac{19}{10}\)
\(D=\left(1+\dfrac{1}{1\cdot3}\right)\cdot\left(1+\dfrac{1}{2\cdot4}\right)\cdot...\cdot\left(1+\dfrac{1}{17\cdot19}\right)\)
\(=\left(1+\dfrac{1}{2^2-1}\right)\cdot\left(1+\dfrac{1}{3^2-1}\right)\cdot...\cdot\left(1+\dfrac{1}{18^2-1}\right)\)
\(=\dfrac{2^2}{2^2-1}\cdot\dfrac{3^2}{3^2-1}\cdot...\cdot\dfrac{18^2}{18^2-1}\)
\(=\dfrac{2\cdot3\cdot...\cdot18}{1\cdot2\cdot...\cdot17}\cdot\dfrac{2\cdot3\cdot...\cdot18}{3\cdot4\cdot...\cdot19}\)
\(=\dfrac{18}{1}\cdot\dfrac{2}{19}=\dfrac{36}{19}\)