Question

[Nguyễn Trúc Mai]

A=\(\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+\frac{1}{13.14}\)

B=\(\frac{1}{20}+\frac{-1}{30}+\frac{1}{40}+\frac{-1}{50}+\frac{1}{60}+\frac{-1}{70}+\frac{1}{80}+\frac{1}{70}+\frac{-1}{60}+\frac{1}{50}+\frac{-1}{40}+\frac{1}{30}+\frac{-1}{20}\)

giúp mình giải các bài toán này nha mình đang càn gấp cảm ơn các bạn nhiều

Answer 1

[Nguyễn Giang Tuấn Nghĩa]

A=\(\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{13}-\frac{1}{14}\)=\(\frac{1}{7}-\frac{1}{14}\)=\(\frac{1}{14}\)

B=0

Answer 2

[Đợi anh khô nước mắt]

\(\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+\frac{1}{13.14}\)

\(=\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+\frac{1}{13}-\frac{1}{14}\)

\(=\frac{1}{7}-\frac{1}{14}=\frac{1}{14}\)