Question

[loan vo]

mong moi nguoi giai giup bai toan

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

"/" la chữ phần nha(giải giúp mjk càng nhanh càng tốt nhá) ths m.n nhìu lúm

Answer 1

[Đinh Đức Hùng]

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

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

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

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

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

Answer 2

[loan vo]

ths bn