bn phaỉ ghi rõ ra chứ!~!

\(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+....+\frac{2}{77.79}\)

=\(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{77}-\frac{1}{79}\)

=\(1-\frac{1}{79}\)

=\(\frac{78}{79}\)

?ko có câu hỏi hat bn!!Lam Thao

?thi sao dau bn!!mk cung vay nk!!nhung moi chuyen van suon se day thoi!!

mk thay chu dep ma!!Trang Seet

???