Question

[Nguyễn Hồ Hoàng Anh]

tính tổng:

A= 6/8+6/56+6/140+...+6/1100+6/1400

mn làm gấp giúp e vs ạ

Answer 1

[ʚƘεŋşɦїŋ ℌїɱʉɾαɞ‏]

ta có A=\(\dfrac{6}{8}\)+\(\dfrac{6}{56}\)+\(\dfrac{6}{140}\)+...+\(\dfrac{6}{1100}\)+\(\dfrac{6}{1400}\)

           =\(\dfrac{3}{4}\)+\(\dfrac{3}{28}\)+\(\dfrac{3}{70}\)+...+\(\dfrac{3}{550}\)+\(\dfrac{3}{700}\)

           =\(\dfrac{3}{1.4}\)+\(\dfrac{3}{4.7}\)+\(\dfrac{3}{7.10}\)+...+\(\dfrac{3}{22.25}\)+\(\dfrac{3}{25.28}\)

           =1-\(\dfrac{1}{4}\)+\(\dfrac{1}{4}\)-\(\dfrac{1}{7}\)+\(\dfrac{1}{7}\)-\(\dfrac{1}{10}\)+...+\(\dfrac{1}{22}\)-\(\dfrac{1}{25}\)+\(\dfrac{1}{25}\)-\(\dfrac{1}{28}\)

          =1-\(\dfrac{1}{28}\)

          =\(\dfrac{27}{28}\)

Vậy A=\(\dfrac{27}{28}\)

Answer 2

[YẾN NGUYỄN]

Ta có: