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![](https://rs.olm.vn/images/avt/0.png?1311)
S = 10/56 + 10/140 + 10/260 + ....... + 10/1400
S = 5/28 + 5/70 + 5/130 + 5/700
3S/5 = 3/4 x 7 + 3/7 x 10 + 30/10 x 13 + ....... + 3/25 x 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 x 5/3
S = 5/14
Vậy S = 5/14
\(S=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+\frac{10}{1400}\)
\(S=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}\)
\(S=\frac{5}{4.7}+\frac{5}{7.10}+\frac{5}{10.13}+...+\frac{5}{25.28}\)
\(S=5.\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{25.28}\right)\)
\(S=5.\left(\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+...+\frac{1}{25.28}\right)\)
\(S=5.\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{25}-\frac{1}{28}\right)\)
\(S=5.\left(\frac{1}{4}-\frac{1}{28}\right)\)
\(S=5.\frac{3}{14}=\frac{15}{14}\)
Vậy \(S=\frac{15}{14}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
M=10/56+10/140+10/260+....+10/1400
M=5/28+5/70+5/130+....+5/700
3M/5=3/4.7+3/7.10+3/10.13+...+3/25.28
3M/5=1/4-1/7+1/7-1/10+1/10-1/13+....+1/25-1/28
3M/5=1/4-1/28
3M5=3/14
M=3/14.5/3
M=5/14
Vậy M=5/14
b)Vì x,y thuộc N suy ra 5y >624 (hình như đề hơi sai, phải là 5y mới đúng)
suy ra 5y có chữ số tận cùng là 5
suy ra 2x có chữ số tận cùng là 1
ta thấy nếu x=0 thì 2x=1,nếu x>0 thì 2x có chữ số tận cùng là chữ số chẵn
mà 2xcó chữ số tận cùng là 1
suy ra x=0
thay vào ta có:20+624=5y
1+624=5y
625=5y
54=5y
suy ra y=4
vậy x=0,y=4
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}\)
\(=\frac{5}{3}\left(\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{25.28}\right)\)
\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}\right)\)
\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{28}\right)\)
\(=\frac{5}{3}\times\frac{3}{14}\)
\(=\frac{5}{14}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
M=5/28+5/70+5/130+...+5/700.
M=5/4*7+5/7*10+5/10*13+...+5/25*58.
3/5*M=3/4*7+3/7*10+3/10*13+...+3/25*28.
3/5*M=1/4-1/28=3/14.
M=3/14:3/5=5/14.
Vậy M=5/14.
M=5/28+5/70+5/130+.....+5/700
M=1/5x(1/4x7+1/7x10+...+1/25x28)
M=1/5x(1/4-1/7+1/7-1/10+....+1./25-1/28)
M=1/5x(1/4-1/28)
M=1/5x3/14
M=3/70
![](https://rs.olm.vn/images/avt/0.png?1311)
B = 10/56 + 10/140 + 10/260 + ...+ 10/1400
B= 5/28 + 5/70 +.....+10/700
= 5/(4.7)+5/(7.10)+....5/(25.28)
3B= 5( 1/4 - 1/7 +1/7-1/10+......+1/25-1/28)
3B = 5 (1/4-1/28)
3B=15/14
B = 15/14 : 3
B = 5/14
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
=\(\frac{5}{3}\times\left(\frac{3}{28}+\frac{3}{70}+\frac{3}{130}+...+\frac{3}{700}\right)\)
=\(\frac{5}{3}\times\left(\frac{3}{4\times7}+\frac{3}{7\times10}+\frac{3}{10\times13}+...+\frac{3}{25\times28}\right)\)
=\(\frac{5}{3}\times\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}\right)\)
=\(\frac{5}{3}\times\left(\frac{1}{4}-\frac{1}{28}\right)\)
=\(\frac{5}{3}\times\frac{3}{14}\)
=\(\frac{5}{14}\)
Ko bít có đúng ko nhưng cứ thử nhé
![](https://rs.olm.vn/images/avt/0.png?1311)
M = 10/56 + 10/140 + 10/260 +...+ 10/1400
M = 5/28 + 5/70 + 5/130 + ... + 5/700
M = 5/4.7 + 5/7.10 + 5/10.13 + ... + 5/25.28
M = 5.1/3.(3/4.7 + 3/7.10 + 3/10.13 + ... + 3/25.28)
M = 5/3.(1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ... + 1/25 - 1/28)
M = 5/3.(1/4 - 1/28)
M = 5/3.3/14
M = 5/14
M = 10/56+10/140+10/260+10/1400
M= 5/28+5/70+5/130+5/700
3M/5=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25
3M/5 = 3/14
M= 3/14+5/3=5/14
Mình ghi nhầm