Tính tổng :
A= 1/20 + 1/30 + 1/42 + 1/56 + ... + 1/930
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AA
1
MH
2 tháng 2 2021
\(A=-\dfrac{1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\\ A=-\left(\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\right)\\ A=-\left(\dfrac{5-4}{4.5}+\dfrac{6-5}{5.6}+\dfrac{7-6}{6.7}+\dfrac{8-7}{7.8}+\dfrac{9-8}{8.9}+\dfrac{10-9}{9.10}\right)\\ A=-\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\right)\\ A=-\left(\dfrac{1}{4}-\dfrac{1}{10}\right)=-\dfrac{3}{20}.\)
29 tháng 6 2023
Sửa đề: A=-1/20+(-1/30)+(-1/42)+(-1/56)+(-1/72)+(-1/90)
=-(1/20+1/30+...+1/90)
=-(1/4-1/5+1/5-1/6+...+1/9-1/10)
=-1/4+1/10
=-5/20+2/20=-3/20
5 tháng 5 2016
A=1/4*5 + 1/5*6 + 1/6*7 +.....+1/99*100
A=1/4-1/5+1/5-1/6+1/6-1/7+...+1/99-1/100
A=1/4-1/100
A=25/100-1/100
A=6/25
5 tháng 5 2016
A = 1/20 + 1/30 + 1/42 + 1/56 + .........+1/990
A = 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + ...........+ 1/99.100
A = 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + ....... + 1/99 - 1/100
A = 1/4 - ( -1/5 + 1/5 ) - ( -1/6 + 1/6 ) - ( -1/7 + 1/7 ) - ...........- ( - 1/99 + 1/99 ) - 1/100
A = 1/4 - 0 - 0 - 0 - ........... - 0 - 1/100
A = 1/4 - 1/100
A = 25/100 - 1/100
A = 24/100
A = 6/25
RM
0
DT
0
NT
2
9 tháng 7 2016
\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{990}\)
\(=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{30.31}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+....+\frac{1}{30}-\frac{1}{31}\)
\(=\frac{1}{4}-\frac{1}{31}\)
\(=\frac{27}{124}\)
PT
1
YN
30 tháng 3 2022
`Answer:`
\(A=-\frac{1}{20}+-\frac{1}{30}+-\frac{1}{42}+-\frac{1}{56}+-\frac{1}{72}+-\frac{1}{90}\)
\(=-\frac{1}{20}-\frac{1}{30}-\frac{1}{42}-\frac{1}{56}-\frac{1}{72}-\frac{1}{90}\)
\(=-\frac{1}{20}-\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(=-\frac{1}{20}-\left(\frac{1}{5}-\frac{1}{10}\right)\)
\(=-\frac{1}{20}-\frac{1}{10}\)
\(=-\frac{3}{20}\)
PN
13
3 tháng 1 2016
A=1/20+1/30+1/42+1/56+1/72+1/90
A=1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10
A=1/4-1/5+1/5-1/6+...+1/9-1/10
A=1/4-1/10
A=3/20
V
1
Q
19 tháng 4 2023
A=\(\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\)
A=\(-\left(\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\right)\)
A=\(-\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\right)\)
A=\(-\left(\dfrac{1}{4}-\dfrac{1}{10}\right)\)
A=\(-\dfrac{3}{20}\)
tách đc như bước 3 là nhờ công thức \(\dfrac{1}{n\left(n+1\right)}=\dfrac{1}{n}-\dfrac{1}{n+1}\) hoặc \(\dfrac{k}{n\left(n+k\right)}=\dfrac{1}{n}-\dfrac{1}{n+k}\) nhé
\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{930}\)
\(=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{30.31}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{30}-\frac{1}{31}\)
\(=\frac{1}{4}-\frac{1}{31}=\frac{27}{124}\)
Vậy \(A=\frac{27}{124}\).
\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{930}\)
\(A=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+...+\frac{1}{30\cdot31}\)
\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{30}-\frac{1}{31}\)
\(A=\frac{1}{4}-\frac{1}{31}\)
\(A=\frac{27}{124}\)