\(\frac{-1}{21}\) + \(\frac{-1}{28}\)
K
Khách
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Q
4
25 tháng 9 2016
3/14:1/28-13/21:1/28+29/42:-1/28-8
=3/14.28-13/21.28+29/42.(-28)-8
=3/14.28-13/21.28+-29/42.28-8
=(3/14-13/21+ -29/42).28-8
=-23/21.28-8
=-92/3-8=-116/3
NA
1
DP
19 tháng 7 2017
Đặt \(A=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}\)
\(A=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+\frac{2}{56}\)
\(A=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}+\frac{2}{7.8}\)
\(A=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{7}-\frac{1}{8}\right)\)
\(A=2\left(\frac{1}{2}-\frac{1}{8}\right)\)
\(\Rightarrow A=2\cdot\frac{3}{8}=\frac{3}{4}\)
AN
2
16 tháng 6 2018
Đặt \(S=\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{1}{78}\)
\(\Rightarrow S=\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{2}{156}\)
\(\Rightarrow S=\frac{2}{6.7}+\frac{2}{7.8}+\frac{2}{8.9}+...+\frac{2}{12.13}\)
\(\Rightarrow S=2.\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{12}-\frac{1}{13}\right)\)
\(\Rightarrow S=2.\left(\frac{1}{6}-\frac{1}{13}\right)\)
\(\Rightarrow S=2.\frac{7}{78}\)
\(\Rightarrow S=\frac{7}{39}\)
LN
16 tháng 6 2018
=2.(1/42+1/56+1/72+...+1/156)
=2.(1/6.7+1/7.8+1/8.9+...+1/12.13)
=2.(1/6-1/13)
=2.(13/78-6/78)
=2.(7/78)
=7/39
chúc bạn học tốt nha
22 tháng 5 2017
\(=1+\frac{1}{1.3}+\frac{1}{3.2}+\frac{1}{2.5}+\frac{1}{5.3}+\frac{1}{3.7}+\frac{1}{7.4}+\frac{1}{4.9}+\frac{1}{9.5}\)
\(=1+1-\frac{1}{5}\)
\(=\frac{10}{5}-\frac{1}{5}\)
\(=\frac{9}{5}\)
Ai thấy đúng thì
AT
3
4 tháng 8 2015
\(\frac{3}{14}:\frac{1}{28}-\frac{13}{21}:\frac{1}{28}-8=\left(\frac{3}{14}-\frac{13}{21}\right):\frac{1}{28}-8=-\frac{17}{42}.28-8=-\frac{34}{3}-8=-\frac{58}{3}\)
4 tháng 8 2015
\(\frac{3}{14}:\frac{1}{28}-\frac{13}{21}:\frac{1}{28}-8\)
\(=\left(\frac{3}{14}-\frac{13}{21}\right):\frac{1}{28}-8\)
\(=-\frac{17}{42}:\frac{1}{28}-8\)
\(=-\frac{34}{3}-8\)
\(=-\frac{58}{3}\)
PV
24 tháng 4 2016
* ĐK: \(x\ne0\)
Đề ra ...<=> \(\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{1}{x\left(x+1\right)}=\frac{2}{9}\)
<=> \(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{2}{x\left(x+1\right)}=\frac{1}{9}\)
<=> \(\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{x\left(x+1\right)}+\frac{1}{x\left(x+1\right)}=\frac{1}{9}\)
<=>\(\frac{1}{6}-\frac{1}{x+1}+\frac{1}{x\left(x+1\right)}=\frac{1}{9}\)
<=>\(\frac{1}{x+1}\left(1-\frac{1}{x}\right)=\frac{1}{6}-\frac{1}{9}\)
<=> \(\frac{x-1}{x\left(x+1\right)}=\frac{1}{36}\)
<=> \(\frac{x-1}{x\left(x-1\right)}=\frac{x-1}{36.\left(x-1\right)}\)
=> x(x-1) = 36. (x-1) => x =36
TN
24 tháng 4 2016
\(\frac{2}{2}.\left(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{2}{x+\left(x+1\right)}\right)=\frac{2}{9}\)
\(2\left(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{x.\left(x+1\right)}\right)=\frac{2}{9}\)
\(\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{x.\left(x+1\right)}=\frac{1}{9}\)
\(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{1}{9}\)
\(\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}\)
\(\frac{1}{x+1}=\frac{1}{6}-\frac{1}{9}\)
\(\frac{1}{x+1}=\frac{1}{18}\)
x+1=18
x=18-1
x=17
LN
4
PT
4 tháng 4 2016
\(S = \frac{1}{3} +\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28} \)
\(S=\frac{1}{3}+\frac{1}{3}.\frac{1}{2}+\frac{1}{5}.\frac{1}{2}+\frac{1}{5}.\frac{1}{3}+\frac{1}{7}.\frac{1}{3}+\frac{1}{7}.\frac{1}{4} \)
\(S=\frac{1}{3}(1+\frac{1}{2})+\frac{1}{5}(\frac{1}{2}+\frac{1}{3})+\frac{1}{7}(\frac{1}{3}+\frac{1}{4})\)
\(S=\frac{1}{3}.\frac{3}{2}+\frac{1}{5}.\frac{5}{6}+\frac{1}{7}.\frac{7}{12}\)
\(S=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}\)
\(S=\frac{6}{12}+\frac{2}{12}+\frac{1}{12}\)
\(S=\frac{9}{12}\)
\(S=\frac{3}{4}\)
24 tháng 4 2016
1/21 + 1/28 + 1/36 + ...+ 1/x(x+1)
=> 2/42 + 2/56 + 2/72 +....+ 2/x(x+1)
=> 2.(1/42 + 1/56 + 1/72 + ... + 1/x.(x+1))
=> 2 .(1/6.7 + 1/7.8 + 1/8.9 + ..+ 1/x.(x+1))
=> 2. ( 1/6 - 1/7 + 1/7-1/8 + ...+ 1/x - 1/x+1
=> 2 . (1/6 - 1/x+1)
=>1/3 - 2/x+1
=\(-\frac{1}{12}\)
hok tốt!
\(-\frac{1}{21}+-\frac{1}{28}=\frac{-28-21}{588}=-\frac{49}{588}=\frac{-1}{12}\)
Chúc bạn học tốt