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TH
2
T1
1 tháng 4 2018
\(=-1\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\right)\)
\(=-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(=-\left(\frac{1}{4}-\frac{1}{10}\right)\)
\(=-\frac{3}{20}\)
SL
2
PA
3 tháng 9 2016
\(A=-\frac{1}{20}-\frac{1}{30}-\frac{1}{42}-\frac{1}{56}-\frac{1}{72}-\frac{1}{90}\)
\(=-\left(\frac{1}{4\times5}+\frac{1}{5\times6}+\frac{1}{6\times7}+\frac{1}{7\times8}+\frac{1}{8\times9}+\frac{1}{9\times10}\right)\)
\(=-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
\(=-\frac{1}{4}+\frac{1}{10}\)
\(=-\frac{3}{20}\)
3 tháng 9 2016
\(A=\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}\)
\(A=-\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\right)\)
\(A=-\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(A=-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
\(A=-\left(\frac{1}{4}-\frac{1}{10}\right)\)
\(A=-\left(\frac{5}{20}-\frac{2}{20}\right)\)
\(A=-\frac{3}{20}\)
BT
0
27 tháng 3 2016
-1/20 - 1/30 - 1/42 - 1/56 - 1/72 - 1/90
= -1/20( 1 + 2/3 + 10/21 + 5/14 + 5/18 + 2/9 )
= -1/20. [ (1 + 2/3) + (10/21 + 5/14) + (5/18 + 2/9)
= -1/20.( 5/3 + 5/6 + 1/2 )
= -1/20.( 5/2 + 1/2 )
= -1/20.3
= -3/20
NM
3
9 tháng 9 2020
a)A=1/20+1/30+1/42+1/56+1/72+1/90+1/110
= 1/4*5 + 1/5*6 + 1/6*7 + ... + 1/10*11
= 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/10 - 1/11
= 1/4 - 1/11
= 7/44
b)B=1/2+1/4+1/6+1/8+...+1/512+1/1024
B = 1/2^1 + 1/2^2 + 1/2^3 + ... + 1/2^9 + 1/2^10
2B = 1 + 1/2 + 1/2^2 + ... + 1/2^10 + 1/2^11
2B - B = B = 1 + 1/2^11
6 tháng 5 2016
A = - (1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 +1/9.10)
A = -( 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10)
A = -( 1/4 - 1/10)
A = -3/20
B = 7(5/2.7 + 4/7.11 + 3/11.14 + 1/14.15 + 13/15.28)
B = 7(1/2 - 1/7 + 1/7 - 1/11 +......+ 1/15 - 1/28)
B = 7(1/2 - 1/28)
B = 7 . 13/28
B = 13/4
DC
4
10 tháng 8 2018
\(B=\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}\)
\(B=-\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\right)\)
\(B=-\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(B=-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
\(B=-\left(\frac{1}{4}-\frac{1}{10}\right)=-\frac{3}{20}\)
10 tháng 8 2018
=>B=\(-\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\right)\)
=>B=\(-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+..+\frac{1}{9}-\frac{1}{10}\right)\)
=>B=\(-\left(\frac{1}{4}-\frac{1}{10}\right)\) =\(\frac{-3}{40}\) HẾTTTTTTTTTTTTT
\(B=\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\\ =\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+\dfrac{1}{8\times9}+\dfrac{1}{9\times10}\\ =\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\\ =\dfrac{1}{4}-\dfrac{1}{10}\\ =\dfrac{10}{40}-\dfrac{4}{40}\\ =\dfrac{6}{40}\\ =\dfrac{3}{20}\)
kẽm ơn bạn :3