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

Vậy S = 5 /14

tk mk nhé !

\(C=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}=10\left(\frac{1}{56}+\frac{1}{140}+\frac{1}{260}+...+\frac{10}{1400}\right)\)

\(C=\frac{10}{2}\left(\frac{1}{28}+\frac{1}{70}+\frac{1}{130}+...+\frac{1}{700}\right)=5\left(\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+\frac{1}{10\cdot13}+...+\frac{1}{25\cdot28}\right)\)

\(C=5\cdot\frac{1}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{25}-\frac{1}{28}\right)\)

\(C=\frac{5}{3}\cdot\left(\frac{1}{4}-\frac{1}{28}\right)=\frac{5}{3}\cdot\frac{3}{14}=\frac{5}{14}\)

\(C=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}=5\left(\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{25.28}\right)=\frac{5}{3}.\left(\frac{1}{4}-\frac{1}{28}\right)=\frac{5}{14}\)

\(C=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+....+\frac{10}{1400}\)

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

\(=\frac{5}{4.7}+\frac{5}{7.10}+\frac{5}{10.13}+....+\frac{5}{25.28}\)

\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+....+\frac{1}{25}-\frac{1}{28}\right)\)

\(=\frac{5}{3}\left(\frac{1}{4}-\frac{1}{28}\right)\)

\(=\frac{5}{3}.\frac{3}{14}=\frac{5}{14}\)

giải giùm với

mơn nhiều

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

B=5/4.7+5/7.10+5/10.13+...+5/25.28

B=5/3.(3/4.7+3/7.10+3/10.13+...+3/25.28)

B=5/3.(1/4-/7+1/7-1/10+1/10+1/10-1/13+...+1/25-1/28)

B=5/3.(1/4-1/28)

B=5/3.3/14

B=5/14

Vậy B=5/14

B=5/3.(3/28+3/70+...+3/700)

B=5/3.(3/4.7+3/7.10+...+3/25.28)

B=5/3.(1/4-1/7+1/7-1/10+...+1/25-1/28)

B=5/3.(1/4-1/28)

B=5/3.3/14

B=5/14

Ko bít có đúng ko, cứ thử đi nhé

E=10/56 + 10/140 + 10/260 +...+10/1400

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

3E/5=3/4.7+3/7.10+3/10.13+...+3/25.28

3E/5=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28

3E/5=1/4-1/28

3E/5=3/14

E=3/14.5/3

E=5/14

Vậy E=5/14

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

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

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

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

\(=\dfrac{5}{3}.\left(\dfrac{1}{4}-\dfrac{1}{28}\right)=\dfrac{5}{14}\)

5/14

\(\)\(\dfrac{10}{56}+\dfrac{10}{140}+...+\dfrac{10}{1400}\)

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

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

\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)\)

\(=\dfrac{5}{3}\cdot\dfrac{6}{28}=2\cdot\dfrac{5}{28}=\dfrac{10}{28}=\dfrac{5}{14}\)

\(=\dfrac{5}{28}+\dfrac{5}{70}+\dfrac{5}{130}+...+\dfrac{5}{700}\\ =\dfrac{5}{3}\left(\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+\dfrac{3}{10\cdot13}+...+\dfrac{3}{25\cdot28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)=\dfrac{5}{3}\cdot\dfrac{3}{14}=\dfrac{5}{14}\)