\(A=\dfrac{5}{28}+\dfrac{5}{70}+\dfrac{5}{130}+...+\dfrac{5}{700}\)

\(\dfrac{3A}{5}=\dfrac{3}{4.7}+\dfrac{3}{7.10}+\dfrac{3}{10.13}+...+\dfrac{3}{25.28}\)

\(\dfrac{3A}{5}=\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\)

\(\dfrac{3A}{5}=\dfrac{1}{4}-\dfrac{1}{28}=\dfrac{3}{14}\)

⇒ \(A=\dfrac{5}{14}\)

bài này bạn chỉ cần tách mẫu ra là làm được thôi

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

Vừa nãy mk k cho bạn đấy,Lê Mai Uyên ạ!

đáp số là 5/14 

Đáp số là\(\frac{5}{14}\)

Bạn nhớ k cho mk nha!
Chúc bạn học giỏi!

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

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

S=5/28+5/70+5/130+.....+5/700

S=5/4x7+5/7x10+5/10x13+.....+5/25x28

Sx3/5=3/4x7+3/7x10+.......+3/25x28

Sx3/5=1/4-1/7+1/7-1/10+1/10+.....+1/25-1/28

Sx3/5=1/4+(1/7-1/7)+(1/9-1/9)+.....+(!/25-1/25)-1/28

Sx3/5=1/4-1/28

Sx3/5=3/14

S=3/14 : 3/5

S=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

khó @gmail.com

\(\frac{5}{14}\)   Là đúng đó bn 

nhớ k cho mình nha! hjhj