Question

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

Answer 1

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

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

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

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

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

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

Answer 2

giải giùm với

Answer 3

mơn nhiều

Answer 4

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

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

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

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

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

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