Tính :
a ) \(S=\frac{1}{3\sqrt{1}+3\sqrt{3}}+\frac{1}{3\sqrt{3}+3\sqrt{5}}+...+\)\(\frac{1}{3\sqrt{2017}+3\sqrt{2019}}\)
b ) \(S=\frac{7}{\sqrt{2.2}+\sqrt{2.3}}+\frac{7}{\sqrt{2.3}+\sqrt{2.4}}\)\(+...+\frac{7}{\sqrt{2.2018}+\sqrt{2.2019}}\)
Những câu hỏi liên quan
tính tổng
\(S=\frac{1}{\sqrt{3}+1}+\frac{1}{\sqrt{5}+\sqrt{3}}+\frac{1}{\sqrt{7}+\sqrt{5}}+...+\frac{1}{\sqrt{2019}+\sqrt{2017}}\)
Bạn ơi cái này mk chỉ ghi cách làm và ct thôi nha
đây dùng hàng đẳng thức (a-b)(a+b)=a^2-b^2
còn kia là công thức toán lớp 6
\(\frac{1}{\sqrt{3}+\sqrt{1}}=\frac{\sqrt{3}-\sqrt{1}}{\left(\sqrt{3}+\sqrt{1}\right)\left(\sqrt{3}-\sqrt{1}\right)}=\frac{\sqrt{3}-\sqrt{1}}{\sqrt{3^2}-\sqrt{1^2}}=\frac{1}{2}\left(\sqrt{3}-\sqrt{1}\right)\)
Tương tự:
\(\frac{1}{\sqrt{5}+\sqrt{3}}=\frac{1}{2}\left(\sqrt{5}-\sqrt{3}\right)\)
.....
\(\frac{1}{\sqrt{2019}+\sqrt{2017}}=\frac{1}{2}\left(\sqrt{2019}-\sqrt{2017}\right)\)
Cộng các vế với nhau ta được:
\(S=\frac{1}{2}\left(\sqrt{2019}-\sqrt{1}\right)=\frac{1}{2}\left(\sqrt{2019}-1\right)\)
Bài này em dùng trục căn thức:
VD: \(\frac{1}{\sqrt{3}+1}=\frac{\sqrt{3}-1}{\left(\sqrt{3}+1\right)\left(\sqrt{3}-1\right)}=\frac{\sqrt{3}-1}{\left(\sqrt{3}\right)^2-1}=\frac{\sqrt{3}-1}{2}\)
Tương tự thì ta có:
\(S=\frac{\sqrt{3}-1}{2}+\frac{\sqrt{5}-\sqrt{3}}{2}+\frac{\sqrt{7}-\sqrt{5}}{2}+...+\frac{\sqrt{2019^2}-\sqrt{2019^2-2}}{2}\)
\(=\frac{\sqrt{2019^2}-1}{2}=\frac{2019-1}{2}=1009\)
Tính :a ) Sfrac{1}{sqrt{1}sqrt{3}}+frac{1}{sqrt{3}+sqrt{5}}+frac{1}{sqrt{5}+sqrt{7}}+.....+frac{1}{sqrt{2017}+sqrt{2019}}b ) Sfrac{1}{sqrt{2}+sqrt{4}}+frac{1}{sqrt{4}+sqrt{6}}+....+frac{1}{sqrt{100}+sqrt{102}}c ) Sfrac{1}{sqrt{1}+sqrt{2}}+frac{1}{sqrt{2}+sqrt{3}}+.....+frac{1}{sqrt{100}+sqrt{101}}d ) Sfrac{1}{sqrt{3}+sqrt{6}}+frac{1}{sqrt{6}+sqrt{9}}+frac{1}{sqrt{9}+sqrt{12}}+....+frac{1}{sqrt{2016}+sqrt{2019}}
Đọc tiếp
Tính :
a ) \(S=\frac{1}{\sqrt{1}\sqrt{3}}+\frac{1}{\sqrt{3}+\sqrt{5}}+\frac{1}{\sqrt{5}+\sqrt{7}}+.....+\)\(\frac{1}{\sqrt{2017}+\sqrt{2019}}\)
b ) \(S=\frac{1}{\sqrt{2}+\sqrt{4}}+\frac{1}{\sqrt{4}+\sqrt{6}}+....+\frac{1}{\sqrt{100}+\sqrt{102}}\)
c ) \(S=\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+.....+\frac{1}{\sqrt{100}+\sqrt{101}}\)
d ) \(S=\frac{1}{\sqrt{3}+\sqrt{6}}+\frac{1}{\sqrt{6}+\sqrt{9}}+\frac{1}{\sqrt{9}+\sqrt{12}}+....+\frac{1}{\sqrt{2016}+\sqrt{2019}}\)
......................?
mik ko biết
mong bn thông cảm
nha ................
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Tính : \(S=\frac{7}{\sqrt{2.2}+\sqrt{2.3}}+\frac{7}{\sqrt{2.3}+\sqrt{2.4}}+.....+\frac{7}{\sqrt{2.2018}+\sqrt{2.2019}}\)
Tính tổng S= \(\frac{1}{1+\sqrt{3}}+\frac{1}{\sqrt{3}+\sqrt{5}}+\frac{1}{\sqrt{5}+\sqrt{7}}+...+\frac{1}{\sqrt{2019^2}+\sqrt{2019^2-2}}\)
\(S=\frac{\sqrt{3}-1}{3-1}+\frac{\sqrt{5}-\sqrt{3}}{5-3}+\frac{\sqrt{7}-\sqrt{5}}{7-5}+...+\frac{\sqrt{2019^2}-\sqrt{2019^2-2}}{2019^2-\left(2019^2-2\right)}\)
\(S=\frac{\sqrt{3}-1}{2}+\frac{\sqrt{5}-\sqrt{3}}{2}+\frac{\sqrt{7}-\sqrt{5}}{2}+...+\frac{\sqrt{2019^2}-\sqrt{2019^2-2}}{2}\)
\(S=\frac{1}{2}\left(\sqrt{3}-1+\sqrt{5}-\sqrt{3}+\sqrt{7}-\sqrt{5}+...+\sqrt{2019^2}-\sqrt{2019^2-2}\right)\)
\(S=\frac{1}{2}\left(-1+\sqrt{2019^2}\right)\)
\(S=\frac{\left(2019-1\right)}{2}=1009\)
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\(S=\frac{1-\sqrt{3}}{1-3}+\frac{\sqrt{3}-\sqrt{5}}{3-5}+\frac{\sqrt{5}-\sqrt{7}}{5-7}+...+\frac{2019-\sqrt{2019^2-2}}{2019^2-2019^2-2}.\)
\(S=\frac{1-\sqrt{3}}{-2}+\frac{\sqrt{3}-\sqrt{5}}{-2}+\frac{\sqrt{5}-\sqrt{7}}{-2}+...+\frac{2019-\sqrt{2019^2-2}}{-2}.\)
\(-2S=1-\sqrt{3}+\sqrt{3}-\sqrt{5}+\sqrt{5}...+2019-\sqrt{2019^2-2}\)
\(-2S=1-\sqrt{2019^2-2}\Rightarrow S=\frac{\sqrt{2019^2-2}-1}{2}\)
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Tính
\(S=\frac{1}{3\sqrt{1}+3\sqrt{3}}+\frac{1}{3\sqrt{3}+3\sqrt{5}}+...+\frac{1}{3\sqrt{2017}+3\sqrt{2019}}\)
\(S=\frac{1}{3\sqrt{1}+3\sqrt{3}}+\frac{1}{3\sqrt{3}+3\sqrt{5}}+...+\frac{1}{3\sqrt{2017}+3\sqrt{2019}}\)
\(S=\frac{1}{3}\left(\frac{1}{\sqrt{1}+\sqrt{3}}+\frac{1}{\sqrt{3}+\sqrt{5}}+...+\frac{1}{\sqrt{2017}+\sqrt{2019}}\right)\)
\(S=\frac{1}{3}\left[\frac{\sqrt{3}-\sqrt{1}}{3-1}+\frac{\sqrt{5}-\sqrt{3}}{5-3}+...+\frac{\sqrt{2019}-\sqrt{2017}}{2019-2017}\right]\)
\(S=\frac{1}{3}\cdot\frac{\sqrt{3}-\sqrt{1}+\sqrt{5}-\sqrt{3}+...+\sqrt{2019}-\sqrt{2017}}{2}\)
\(S=\frac{\sqrt{2019}-\sqrt{1}}{6}\)
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\(2S=\frac{1}{3}\left(\frac{2}{\sqrt{1}+\sqrt{3}}+\frac{2}{\sqrt{3}+\sqrt{5}}+...+\frac{2}{\sqrt{2017}+\sqrt{2019}}\right)\)
\(2S=\frac{1}{3}\left(\frac{3-1}{\sqrt{1}+\sqrt{3}}+\frac{5-3}{\sqrt{3}+\sqrt{5}}+...+\frac{2019-2017}{\sqrt{2017}+\sqrt{2019}}\right)\)
\(2S=\frac{1}{3}\left(\frac{\left(\sqrt{3}-1\right)\left(\sqrt{3}+1\right)}{\sqrt{3}+1}+\frac{\left(\sqrt{5}-\sqrt{3}\right)\left(\sqrt{5}+\sqrt{3}\right)}{\sqrt{3}+\sqrt{5}}+...+\frac{\left(\sqrt{2019}-\sqrt{2017}\right)\left(\sqrt{2019}+\sqrt{2017}\right)}{\sqrt{2019}+\sqrt{2017}}\right)\)
\(2S=\frac{1}{3}\left(\sqrt{3}-1+\sqrt{5}-\sqrt{3}+...+\sqrt{2019}-\sqrt{2017}\right)\)
\(S=\frac{\sqrt{2019}-1}{6}\)
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help me !
tính S = \(\frac{1}{3+\sqrt{3}}+\frac{1}{3\sqrt{5}+5\sqrt{3}}+\frac{1}{5\sqrt{7}+\sqrt{5}7}+.....+\frac{1}{101\sqrt{103}+103\sqrt{101}}\text{ [}\)!
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 : \(S=\frac{1}{3\sqrt{1}+1\sqrt{3}}+\frac{1}{3\sqrt{5}+5\sqrt{3}}+\frac{1}{5\sqrt{7}+7\sqrt{5}}+...+\frac{1}{101\sqrt{103}+103\sqrt{101}}\)
\(=\frac{1}{2}\left(\frac{1}{\sqrt{1}}-\frac{1}{\sqrt{3}}\right)+\frac{1}{2}\left(\frac{1}{\sqrt{3}}-\frac{1}{\sqrt{5}}\right)+\frac{1}{2}\left(\frac{1}{\sqrt{5}}-\frac{1}{\sqrt{7}}\right)+...+\frac{1}{2}\left(\frac{1}{\sqrt{101}}-\frac{1}{\sqrt{103}}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{\sqrt{3}}+\frac{1}{\sqrt{3}}-\frac{1}{\sqrt{5}}+\frac{1}{\sqrt{5}}-\frac{1}{\sqrt{7}}+...+\frac{1}{\sqrt{101}}-\frac{1}{\sqrt{103}}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{\sqrt{103}}\right)\)
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Tất cả bằng 1 tin đi
Xem thêm câu trả lời
Tínha/left(frac{2sqrt{3}-sqrt{6}}{sqrt{8}-2}-frac{sqrt{216}}{3}right).frac{1}{sqrt{6}}b/left(frac{5}{4-sqrt{11}}+frac{1}{3+sqrt{7}}-frac{6}{sqrt{7}-2}-frac{sqrt{7}-5}{2}right)c/left(frac{sqrt{14}-sqrt{7}}{1-sqrt{2}}+frac{sqrt{15}-sqrt{5}}{1-sqrt{3}}right):frac{1}{sqrt{7}-sqrt{5}}d/frac{sqrt{5}-sqrt{3}}{sqrt{5}+sqrt{3}}+frac{sqrt{5}+sqrt{3}}{sqrt{5}-sqrt{3}}-frac{sqrt{5}+1}{sqrt{5}-1}
Đọc tiếp
Tính
a/\(\left(\frac{2\sqrt{3}-\sqrt{6}}{\sqrt{8}-2}-\frac{\sqrt{216}}{3}\right).\frac{1}{\sqrt{6}}\)
b/\(\left(\frac{5}{4-\sqrt{11}}+\frac{1}{3+\sqrt{7}}-\frac{6}{\sqrt{7}-2}-\frac{\sqrt{7}-5}{2}\right)\)
c/\(\left(\frac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\frac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}\right):\frac{1}{\sqrt{7}-\sqrt{5}}\)
d/\(\frac{\sqrt{5}-\sqrt{3}}{\sqrt{5}+\sqrt{3}}+\frac{\sqrt{5}+\sqrt{3}}{\sqrt{5}-\sqrt{3}}-\frac{\sqrt{5}+1}{\sqrt{5}-1}\)
1. Tìm số tự nhiên n sao cho :
dfrac{1}{1.2}+dfrac{1}{2.3}+dfrac{1}{3.4}+..+dfrac{1}{n.left(n+1right)}dfrac{2999}{3000}
2. Tính :
a ) S2018.3+2018.4+2018.5+...+2018.2018
b ) dfrac{1}{sqrt{8}+sqrt{10}}+dfrac{1}{sqrt{10}+sqrt{12}}+dfrac{1}{sqrt{12}+sqrt{14}}+...+dfrac{1}{sqrt{200}+sqrt{202}}
c ) S5.21^2+5.21^3+5.21^4+....+5.21^{2018}
d ) A9+99+999+9999+...+9..9( 99 chữ số 9)
e ) 72+772+7772+...+77...72( 77 chữ số 7 )
2. Tính tổng :
a ) Sdfrac{1}{3sqrt{1}+3sqrt{3}}+dfrac{1}{3sqrt{3}+3sqrt{...
Đọc tiếp
1. Tìm số tự nhiên n sao cho :
\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+..+\dfrac{1}{n.\left(n+1\right)}=\dfrac{2999}{3000}\)
2. Tính :
a ) \(S=2018.3+2018.4+2018.5+...+2018.2018\)
b ) \(\dfrac{1}{\sqrt{8}+\sqrt{10}}+\dfrac{1}{\sqrt{10}+\sqrt{12}}+\dfrac{1}{\sqrt{12}+\sqrt{14}}+...+\dfrac{1}{\sqrt{200}+\sqrt{202}}\)
c ) \(S=5.21^2+5.21^3+5.21^4+....+5.21^{2018}\)
d ) \(A=9+99+999+9999+...+9..9\)( 99 chữ số 9)
e ) 72+772+7772+...+77...72( 77 chữ số 7 )
2. Tính tổng :
a ) \(S=\dfrac{1}{3\sqrt{1}+3\sqrt{3}}+\dfrac{1}{3\sqrt{3}+3\sqrt{5}}+...+\dfrac{1}{3\sqrt{2017}+3\sqrt{2019}}\)
b ) S = \(\dfrac{1}{\sqrt{2.2}+\sqrt{2.3}}+\dfrac{1}{\sqrt{2.3}+\sqrt{2.4}}+\dfrac{1}{\sqrt{2.4}+\sqrt{2.5}}+...+\dfrac{1}{\sqrt{2.2018}+\sqrt{2.2019}}\)
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
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help me !
tính S = \(\frac{1}{3+\sqrt{3}}+\frac{1}{3\sqrt{5}+5\sqrt{3}}+\frac{1}{5\sqrt{7}+\sqrt{5}7}+.....+\frac{1}{101\sqrt{103}+103\sqrt{101}}\text{Doumo arigatou}\)!
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 )