Question

tính nhanh các tổng    \(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}...+\frac{1}{99}+\frac{1}{110}\)

Answer 1

\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{110}=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{10.11}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{10}-\frac{1}{11}\)

\(=\frac{1}{2}-\frac{1}{11}\)

\(\frac{9}{22}\)

Answer 2

Hồ Thu Giang làm đúng ròi

Answer 3

Đề sai phải không bạn, phải là\(...+\frac{1}{90}+\frac{1}{110}\)chứ

Answer 4

9/22 nhé bạn

Answer 5

NẾU THAY 1/90 =1/50  THÌ BẰNG BAO NHIÊU

Answer 6

\(\frac{9}{2}\)

Answer 7

\(=\frac{9}{22}\)

Answer 8

= 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7 + ... + 1/9x10 + 1/10x11

= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/9 - 1/10 + 1/10 - 1/11

= 1/2 - 1/11

= 11/22 - 2/22

= 9/22