Tính tổng: M=10/56+10/140+10/260+...+10/1400
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ND
0
NP
18
17 tháng 1 2016
M=5/28+5/70+...+5/700=5/4.7+5/7.10+...+5/25.28=>3M=5(1/4-1/7+1/7-1/10+...+1/25-1/28)
=>3M=5(1/4-1/28)=>3M=15/14=>M=5/14
HT
6 tháng 3 2016
Đầu tiên rút gọn M trước
M= 5/28 + 5/70 +.....+10/700
= 5/(4.7)+5/(7.10)+....5/(25.28)
3M= 5( 1/4 - 1/7 +1/7-1/10+......+1/25-1/28)
3M= 5 (1/4-1/28)
3M=15/14
M= 5/14 :D
PD
1
25 tháng 4 2016
Đặt S= 5/28 + 5/70 + 5/130 +.....+5/700
= 5/4.7 + 5/7.10 + ......+ 5/ 25.28
3S/5=1/4-1/7+1/7-1/10+.....+1/25-1/28
3S/5=1/4-1/28
3S/5=3/14
3S=3/14x5
3S=15/14
S=15/14x1/3
S=15/42
PN
15
12 tháng 4 2015
S=10/56+10/140+10/260+....+10/1400
S=5/28+5/70+5/130+....+5/700
3S/5=3/4.7+3/7.10+3/10.13+...+3/25.28
3S/5=1/4-1/7+1/7-1/10+1/10-1/13+....+1/25-1/28
3S/5=1/4-1/28
3S/5=3/14
S=3/14.5/3
S=5/14
Vậy S=5/14
16 tháng 4 2015
S=10/56+10/140+10/260+...........+10/1400
S=5/28+5/70+5/130+........+5/700
3S/5=3/4.7+3/7.10+3/13.10+.........+3/25.28
3S/5=1/4-1/7+1/7-1/10+1/10-1/13+.........+1/25-1/28
3S/5=1/4-1/28
3S/5=3/14
S=3/14.5/3
S=5/14
25 tháng 1 2022
\(A=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
\(A=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}\)
\(\frac{3A}{5}=\frac{3}{4\times7}+\frac{3}{7\times10}+\frac{3}{10\times13}+...+\frac{3}{25\times28}\)
\(\frac{3A}{5}=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{25}-\frac{1}{28}\)
\(\frac{3A}{5}=\frac{1}{4}-\frac{1}{28}\)
\(\frac{3A}{5}=\frac{3}{14}\)
\(A=\frac{3}{14}\times\frac{5}{3}\)
\(A=\frac{5}{14}\)
US
0
NN
1
ND
0
CH
19 tháng 2 2019
\(A=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+..+\frac{10}{1400}\)
\(A=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+..+\frac{5}{700}\)
\(A.\frac{3}{5}=\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+..+\frac{3}{25.28}\)
\(A.\frac{3}{5}=\frac{7-4}{4.7}+\frac{10-7}{7.10}+\frac{10-13}{10.13}+..+\frac{28-15}{25.28}\)
\(A.\frac{3}{5}=\frac{7}{4.7}-\frac{4}{4.7}+\frac{10}{7.10}-\frac{7}{7.10}+\frac{13}{13.10}+...+\frac{25}{25.28}-\frac{28}{25.28}\)
\(A.\frac{3}{5}=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}\)
\(A.\frac{3}{5}=\frac{1}{4}-\frac{1}{28}=\frac{6}{28}=\frac{3}{14}\)
\(A=\frac{3}{14}:\frac{3}{5}=\frac{5}{14}\)
M=10/56+10/140+10/260+...+10/1400
=5/28+5/70+5/130+...+5/700
=5/4.7+5/7.10+5/10.13+...+5/25.28
=5/3(3/4.7+3/7.10+3/10.13+...+3/25.28)
=5/3(1/4-1/7+1/7-1/10+1/10-1/1+...+1/25-1/28)
=5/3.(1/4-1/28)
=5/3.3/14
=5/14
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