Question

[cao trung hieu]

A = \(\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+...+\frac{1}{18.19}+\frac{1}{19.20}\)

B = \(\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{17.19}+\frac{2}{19.21}\)

giải giúp nhé ! thank you

2 like cho ai đấy đủ .

Answer 1

[Nguyễn Đình Dũng]

a) A = 1/7.8 + 1/8.9 + 1/9.10 + ... + 1/18.19 + 1/19.20

A = 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + ... + 1/18 - 1/19 + 1/19 - 1/20

A = 1/7 - 1/20

A = 13/140

b) B 2/5.7 + 2/7. 9 + 2/9.11 + .... + 2/17.19 + 2/19.21

B = 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + ... + 1/17 - 1/19 + 1/19 - 1/21

B = 1/5 - 1/21

B = 16/105

Answer 2

[Nguyễn Ngọc Quý]

A = \(\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{29.20}=\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-...-\frac{1}{20}\)

\(=\frac{1}{7}-\frac{1}{20}=\frac{13}{140}\)