tính tổng :
S = (5^2 / 1x6) + (5^2 / 6x11) +( 5^2 / 11x16) + (5^2 / 16x21) + (5^2 / 21x26)
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Q
16 tháng 4 2017
\(\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}\)
\(=5^2\left(\frac{1}{1.6}+\frac{1}{6.11}+\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}\right)\)
\(=25.\frac{1}{5}\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{21}-\frac{1}{26}\right)\)
\(=5\left(1-\frac{1}{26}\right)\)
\(=5.\frac{25}{26}\)
\(=\frac{125}{26}\)
27 tháng 4 2019
\(S=\frac{5^2}{1\cdot6}+\frac{5^2}{6\cdot11}+\frac{5^2}{11\cdot16}+\frac{5^2}{16\cdot21}+\frac{5^2}{21\cdot26}\)
\(S=\frac{25}{1\cdot6}+\frac{25}{6\cdot11}+\frac{25}{11\cdot16}+\frac{25}{16\cdot21}+\frac{25}{21\cdot26}\)
\(S=5\left[\frac{5}{1\cdot6}+\frac{5}{6\cdot11}+\frac{5}{11\cdot16}+\frac{5}{16\cdot21}+\frac{5}{21\cdot26}\right]\)
\(S=5\left[1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{21}-\frac{1}{26}\right]\)
\(S=5\left[1-\frac{1}{26}\right]=5\cdot\frac{25}{26}=\frac{125}{26}\)
27 tháng 4 2019
Bài làm
S = \(\frac{5^2}{1.6}\)+ \(\frac{5^2}{6.11}\)+ \(\frac{5^2}{11.16}\)+ \(\frac{5^2}{16.21}\)+\(\frac{5^2}{21.26}\)
S : 5 = \(\frac{5}{1.6}\)+ \(\frac{5}{6.11}\)+ \(\frac{5}{11.16}\) + \(\frac{5}{16.21}\) + \(\frac{5}{21.26}\)
S : 5 = 1 - \(\frac{1}{6}\)+ \(\frac{1}{6}\)- \(\frac{1}{11}\) + \(\frac{1}{11}\)- \(\frac{1}{16}\)+ \(\frac{1}{16}\)- \(\frac{1}{21}\)+ \(\frac{1}{21}\)- \(\frac{1}{26}\)
S : 5 = 1 - \(\frac{1}{26}\)
S : 5 = \(\frac{25}{26}\)
S = \(\frac{125}{26}\)
NM
0
NP
1
Q
23 tháng 4 2017
\(S=\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}\)
\(5S=5^2\left(\frac{1}{1.6}+\frac{1}{6.11}+\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}\right)\)
\(5S=5^2\left(1-\frac{1}{26}\right)\)
\(\Rightarrow S=5^2:5\left(1-\frac{1}{26}\right)\)
\(S=5\left(1-\frac{1}{26}\right)\)
\(S=5.\frac{25}{26}\)
\(S=\frac{125}{26}\)
DD
1
25 tháng 7 2016
\(\frac{2}{1x6}+\frac{2}{6x11}+\frac{2}{11x16}+\frac{2}{16x21}+\frac{2}{21x26}\)
= \(\frac{2}{6}+\frac{2}{66}+\frac{2}{176}+\frac{2}{336}+\frac{2}{546}\)
= \(\frac{1}{3}+\frac{1}{33}+\frac{1}{88}+\frac{1}{168}+\frac{1}{273}\)
=\(\frac{5}{13}\)
Mình tự nghĩ đấy .
Chúc bạn học tốt!
7 tháng 5 2020
Lớp 4 đó nếu ai ko làm được thì ko phải học sinh giỏi đó nha
H9
HT.Phong (9A5)
CTVHS
19 tháng 10 2023
a) \(\dfrac{5}{1\cdot6}+\dfrac{5}{6\cdot11}+...+\dfrac{5}{26\cdot31}\)
\(=1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+...+\dfrac{1}{26}-\dfrac{1}{31}\)
\(=1-\dfrac{1}{31}\)
\(=\dfrac{30}{31}\)
b) \(\dfrac{4}{11\cdot16}+\dfrac{4}{16\cdot21}+...+\dfrac{4}{61\cdot66}\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{5}{11\cdot16}+\dfrac{5}{16\cdot21}+...+\dfrac{5}{61\cdot66}\right)\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-...+\dfrac{1}{61}-\dfrac{1}{66}\right)\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{1}{11}-\dfrac{1}{66}\right)\)
\(=\dfrac{4}{5}\cdot\dfrac{5}{66}\)
\(=\dfrac{4}{66}\)
\(=\dfrac{2}{33}\)
KV
19 tháng 10 2023
a) A = 5²/(1.6) + 5²/(6.11) + ... + 5²/(26.31)
= 5.[5/(1.6) + 5/(6.11) + ...+ 5/(26.31)]
= 5.(1 - 1/6 + 1/6 - 1/11 + ... + 1/26 - 1/31)
= 5.(1 - 1/31)
= 5.30/31
= 150/31
b) B = 4/(11.16) + 4/(16.21) + ... + 4/(61.66)
= 4/5 .[5/(11.16) + 5/(16.21) + ... + 5/(61.66)]
= 4/5.(1/11 - 1/16 + 1/16 - 1/21 + ... + 1/61 - 1/66)
= 4/5.(1/11 - 1/66)
= 4/5 . 5/66
= 2/33
QM
2
NT
Nguyễn Thị Thương Hoài
Giáo viên
VIP
27 tháng 4 2023
A = \(\dfrac{25}{1\times6}\) + \(\dfrac{25}{6\times11}\) + \(\dfrac{25}{11\times16}\)+\(\dfrac{25}{16\times21}\)+ \(\dfrac{25}{26\times31}\)
A = 5 \(\times\) ( \(\dfrac{5}{1\times6}\)+\(\dfrac{5}{6\times11}\)+\(\dfrac{5}{11\times16}\)+\(\dfrac{5}{16\times21}\)+\(\dfrac{5}{26\times31}\))
A = 5 \(\times\) ( \(\dfrac{1}{1}\) - \(\dfrac{1}{6}\) + \(\dfrac{1}{6}\) - \(\dfrac{1}{11}\)+ \(\dfrac{1}{11}\)- \(\dfrac{1}{16}\)+ \(\dfrac{1}{16}\)- \(\dfrac{1}{21}\)+ \(\dfrac{1}{26}\)- \(\dfrac{1}{31}\))
A = 5 \(\times\)( 1 - \(\dfrac{1}{31}\))
A = 5 \(\times\) \(\dfrac{30}{31}\)
A = \(\dfrac{150}{31}\)