Tính \(\frac{-1}{21}+\frac{-1}{28};\frac{-5}{12}+0,75\)
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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
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
\(\frac{3}{14}:\frac{1}{28}-\frac{13}{21}:\frac{1}{28}-8\)
\(=\left(\frac{3}{14}-\frac{13}{21}\right):\frac{1}{28}-8\)
\(=-\frac{17}{42}:\frac{1}{28}-8\)
\(=-\frac{34}{3}-8\)
\(=-\frac{58}{3}\)
LN
4
PT
4 tháng 4 2016
\(S = \frac{1}{3} +\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28} \)
\(S=\frac{1}{3}+\frac{1}{3}.\frac{1}{2}+\frac{1}{5}.\frac{1}{2}+\frac{1}{5}.\frac{1}{3}+\frac{1}{7}.\frac{1}{3}+\frac{1}{7}.\frac{1}{4} \)
\(S=\frac{1}{3}(1+\frac{1}{2})+\frac{1}{5}(\frac{1}{2}+\frac{1}{3})+\frac{1}{7}(\frac{1}{3}+\frac{1}{4})\)
\(S=\frac{1}{3}.\frac{3}{2}+\frac{1}{5}.\frac{5}{6}+\frac{1}{7}.\frac{7}{12}\)
\(S=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}\)
\(S=\frac{6}{12}+\frac{2}{12}+\frac{1}{12}\)
\(S=\frac{9}{12}\)
\(S=\frac{3}{4}\)
BG
0
NL
3
LT
16 tháng 4 2017
p=1-1/2.5-1/3.5-1/1.3-1/4.7-1/2.3-1/3.7
p=1-(1/2.1/5-1/3.1/5)-(1/1.1/3-1/2.1/3)-(1/4.1/7-1/3.1/7)
p=1-(1/5.(1/2-1/3))-(1/3.(1-1/2))-(1/7.(1/4-1/3)
p=1-(1/5.1/6)-(1/3.1/2)-(1/7.-1/12)
p=1-1/30-1/6+1/84
p=341/420
26 tháng 8 2016
\(\left(\frac{-1}{4}+\frac{7}{33}-\frac{5}{3}\right)-\left(\frac{-5}{4}+\frac{6}{11}-\frac{48}{49}\right)=\left(\frac{-1}{4}-\frac{16}{11}\right)-\left(-\frac{31}{44}-\frac{48}{49}\right)=-\frac{1}{4}-\frac{16}{11}+\frac{31}{44}+\frac{48}{49}=-\frac{1}{49}\)
NQ
4
BN
19 tháng 5 2016
\(M=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{946}+\frac{1}{990}\)
\(M=\frac{2}{30}+\frac{2}{42}+...+\frac{2}{1980}\)
\(M=2\left(\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{44.45}\right)\)
\(M=2\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{44}-\frac{1}{45}\right)\)
\(M=2\left(\frac{1}{5}-\frac{1}{45}\right)\)
\(M=2\times\frac{8}{45}\)
\(M=\frac{16}{45}\)
PA
19 tháng 5 2016
\(M=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{946}+\frac{1}{990}\)
\(M=\frac{1\times2}{15\times2}+\frac{1\times2}{21\times2}+\frac{1\times2}{28\times2}+\frac{1\times2}{946\times2}+\frac{1\times2}{990\times2}\)
\(M=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+...+\frac{2}{1892}+\frac{2}{1980}\)
\(M=2\times\left(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{1892}+\frac{1}{1980}\right)\)
\(M=2\times\left(\frac{1}{5\times6}+\frac{1}{6\times7}+\frac{1}{7\times8}+...+\frac{1}{43\times44}+\frac{1}{44\times45}\right)\)
\(M=2\times\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{43}-\frac{1}{44}+\frac{1}{44}-\frac{1}{45}\right)\)
\(M=2\times\left(\frac{1}{5}-\frac{1}{45}\right)\)
\(M=2\times\left(\frac{9}{45}-\frac{1}{45}\right)\)
\(M=2\times\frac{8}{45}\)
\(M=\frac{16}{45}\)
Chúc bạn học tốt
18 tháng 1 2017
lấy (1/3 + 1/15 +1/10 + 1/21 ) + (1/36 + 1/28 + 1/6) + (1/45 + 1/55)
= (4/50 + 3/70) + 2/100
= 7/120 + 2/100
= 9/220
NH
1
21 tháng 4 2016
- 2.s= 1/30+1/42+1/56+...+ 1/380
2.S= 1/ 5.6 =1/ 6.7 +1/ 7.8 +...+1/ 19.20
2.S= 1/5-1/20
2S= 3/20
\(-\frac{1}{21}+-\frac{1}{28}=-\frac{1}{21}-\frac{1}{28}=-\frac{4}{84}-\frac{3}{84}=-\frac{7}{84}=-\frac{1}{12}\)
\(-\frac{5}{12}+\frac{3}{4}=-\frac{5}{12}+\frac{9}{12}=\frac{4}{12}=\frac{1}{3}\)