Tính : \(S=\frac{7}{\sqrt{2.2}+\sqrt{2.3}}+\frac{7}{\sqrt{2.3}+\sqrt{2.4}}+.....+\frac{7}{\sqrt{2.2018}+\sqrt{2.2019}}\)
K
Khách
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Những câu hỏi liên quan
13 tháng 7 2022
Câu 1:
\(\Leftrightarrow1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{n}-\dfrac{1}{n+1}=\dfrac{2999}{3000}\)
\(\Leftrightarrow1-\dfrac{1}{n+1}=\dfrac{2999}{3000}\)
=>n+1=3000
hay n=2999
NV
Nguyễn Việt Lâm
Giáo viên
19 tháng 10 2019
\(\sqrt{1+\frac{8n^2-1}{\left(2n-1\right)^2\left(2n+1\right)^2}}=\sqrt{1+\frac{8n^2-1}{\left(4n^2-1\right)^2}}=\sqrt{\frac{\left(4n^2-1\right)^2+8n^2-1}{\left(4n^2-1\right)^2}}\)
\(=\sqrt{\frac{16n^4-8n^2+1+8n^2-1}{\left(4n^2-1\right)^2}}=\frac{4n^2}{4n^2-1}=1+\frac{1}{4n^2-1}=1+\frac{1}{2}\left(\frac{1}{2n-1}-\frac{1}{2n+1}\right)\)
\(\Rightarrow S=1009+\frac{1}{2}\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{2017}-\frac{1}{2019}\right)\)
\(=1009+\frac{1}{2}\left(1-\frac{1}{2019}\right)=...\)
6 tháng 8 2016
Xét biểu thức phụ : \(\frac{1}{\left(2n+3\right)\sqrt{2n+1}+\left(2n+1\right)\sqrt{2n+3}}=\frac{1}{\sqrt{2n+1}.\sqrt{2n+3}\left(\sqrt{2n+1}+\sqrt{2n+3}\right)}\)
\(=\frac{\sqrt{2n+3}-\sqrt{2n+1}}{\sqrt{2n+1}.\sqrt{2n+3}\left[\left(2n+3\right)-\left(2n+1\right)\right]}\)
\(=\frac{\sqrt{2n+3}-\sqrt{2n+1}}{2\sqrt{2n+1}.\sqrt{2n+3}}=\frac{1}{2}\left(\frac{1}{\sqrt{2n+1}}-\frac{1}{\sqrt{2n+3}}\right)\)với \(n\ge1\)
Áp dụng : \(S=\frac{1}{3\sqrt{1}+1\sqrt{3}}+\frac{1}{3\sqrt{5}+5\sqrt{3}}+\frac{1}{5\sqrt{7}+7\sqrt{5}}+...+\frac{1}{101\sqrt{103}+103\sqrt{101}}\)
\(=\frac{1}{2}\left(\frac{1}{\sqrt{1}}-\frac{1}{\sqrt{3}}\right)+\frac{1}{2}\left(\frac{1}{\sqrt{3}}-\frac{1}{\sqrt{5}}\right)+\frac{1}{2}\left(\frac{1}{\sqrt{5}}-\frac{1}{\sqrt{7}}\right)+...+\frac{1}{2}\left(\frac{1}{\sqrt{101}}-\frac{1}{\sqrt{103}}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{\sqrt{3}}+\frac{1}{\sqrt{3}}-\frac{1}{\sqrt{5}}+\frac{1}{\sqrt{5}}-\frac{1}{\sqrt{7}}+...+\frac{1}{\sqrt{101}}-\frac{1}{\sqrt{103}}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{\sqrt{103}}\right)\)
6 tháng 3 2021
Xét biểu thức phụ :
1
(2n+3)√2n+1+(2n+1)√2n+3 =
1
√2n+1.√2n+3(√2n+1+√2n+3)
=
√2n+3−√2n+1
√2n+1.√2n+3[(2n+3)−(2n+1)]
=
√2n+3−√2n+1
2√2n+1.√2n+3 =
1
2 (
1
√2n+1 −
1
√2n+3 )với
n≥1
Áp dụng :
S=
1
3√1+1√3 +
1
3√5+5√3 +
1
5√7+7√5 +...+
1
101√103+103√101
=
1
2 (
1
√1 −
1
√3 )+
1
2 (
1
√3 −
1
√5 )+
1
2 (
1
√5 −
1
√7 )+...+
1
2 (
1
√101 −
1
√103 )
=
1
2 (1−
1
√3 +
1
√3 −
1
√5 +
1
√5 −
1
√7 +...+
1
√101 −
1
√103 )
=
1
2 (1−
1
√103 )
9 tháng 7 2019
\(\frac{\sqrt{7}+7}{\sqrt{7}+1}-\frac{\sqrt{7}-\sqrt{14}}{\sqrt{2}-1}+\frac{2\sqrt{35}-2\sqrt{7}}{1-\sqrt{5}}\)
\(=\frac{\sqrt{7}\left(1+\sqrt{7}\right)}{\sqrt{7}+1}-\frac{\sqrt{7}\left(1-\sqrt{2}\right)}{\sqrt{2}-1}+\frac{2\sqrt{7}\left(\sqrt{5}-1\right)}{1-\sqrt{5}}\)
\(=\frac{\sqrt{7}\left(1+\sqrt{7}\right)}{\sqrt{7}+1}+\frac{\sqrt{7}\left(\sqrt{2}-1\right)}{\sqrt{2}-1}-\frac{2\sqrt{7}\left(\sqrt{5}-1\right)}{\sqrt{5}-1}\)
\(=\sqrt{7}+\sqrt{7}-2\sqrt{7}\)
\(=0\)
