a<-1/3

ngáo vãi tao bít giải rồi mà mày phải trình bày

\(D=\dfrac{10}{56}+\dfrac{10}{140}+\dfrac{10}{260}+...+\dfrac{10}{1400}\)

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

\(=5\left(\dfrac{1}{28}+\dfrac{1}{70}+\dfrac{1}{130}+...+\dfrac{1}{700}\right)\)

\(=5\left(\dfrac{1}{4.7}+\dfrac{1}{7.10}+\dfrac{1}{10.13}+...+\dfrac{1}{25.28}\right)\)

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

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

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

\(=\dfrac{5}{14}\)

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

\(=\dfrac{5}{3}\left(\dfrac{1}{4}-\dfrac{1}{28}\right)=\dfrac{5}{3}.\dfrac{6}{28}=\dfrac{5}{14}\)

\(E=\dfrac{2}{2.3}+\dfrac{2}{3.4}+\dfrac{2}{4.5}+...+\dfrac{2}{24.25}=2\left(\dfrac{1}{2}-\dfrac{1}{25}\right)=\dfrac{2.23}{50}=\dfrac{23}{25}.\)

\(\dfrac{D}{E}=\dfrac{5}{24}.\dfrac{25}{23}=\dfrac{125}{552}.\)

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

H=10/56+10/1400

H=13/70

vay H=13/70

ai di qua nho h cho minh nhe

SAO MÀ DỐT THẾ ! DỄ THẾ NÀY MÀ LÀM KHÔNG ĐƯỢC

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

H = 5/3 × (3/28 + 3/70 + 3/130 + ... + 3/700)

H = 5/3 × (3/4×7 + 3/7×10 + 3/10×13 + ... + 3/25×28)

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

H = 5/3 × (1/4 - 1/28)

H = 5/3 × 3/14

H = 5/14

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

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

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

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

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

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

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

\(=\frac{5}{3}\times\left(\frac{3}{28}+\frac{3}{70}+....+\frac{3}{700}\right)\)

\(=\frac{5}{3}\times\left(\frac{3}{4}\times7+\frac{3}{7}\times10+.....+\frac{3}{25}\times28\right)\)

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

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

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

H = 10/56 + 10/140 + 10/260 +…..+ 10/1400
H = 5/28 + 5/70 + 5/130 +…..+ 5/700
H = 5 x (1/4.7 + 1/7.10 + 1/10.13 +…………+ 1/25.28)
H = 5/3 x (3/4.7 + 3/7.10 + 3/10.13 +………+ 3/25.28)
H = 5/3 x (1/4-1/7+1/7-1/10+1/10-1/13+……..+1/25-1/28)
H = 5/3 x (1/4-1/28)
H = 5/3 x 3/14 = 5/14 

= 5/ 14

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