vãi nồi :
S=7/10.11+7/11.12+...+7/69.70
=>S= 7.(1/10.11+1/11.12+...+1/69.70)
=> S= 7. ( 1/10-1/11+...+1/69-1/70)
=> S= 7. ( 1/10 - 1/70)
=> S = 7. 3/35
=> S = 3/5
We are one_Nguyễn Ngọc Sáng
\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
\(=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)
\(=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(=7.\frac{3}{35}\)
\(=\frac{3}{5}\)
\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
\(=7\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)
\(=7\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(=7\left(\frac{1}{10}-\frac{1}{70}\right)=7\left(\frac{7}{70}-\frac{1}{70}\right)=7.\frac{6}{70}=\frac{6}{10}=\frac{3}{5}\)
We are one_Nguyễn Ngọc Sáng t lại sau m
\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
\(=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)
\(=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(=7.\frac{3}{35}\)
\(=\frac{3}{5}\)
\(=7\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(=7\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(=7\times\frac{3}{35}\)
\(=\frac{3}{5}\)
mấy chế ăn cái j mà nhanh thế
Đặt biểu thức trên = A. Ta có:
\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
\(A=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(A=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(A=7.\frac{3}{35}\)
\(A=\frac{3}{5}\)
đặt A = \(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
=> \(\frac{A}{7}\)=\(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\)
\(\frac{A}{7}=\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\)
\(\frac{A}{7}=\frac{1}{10}-\frac{1}{70}=\frac{7}{70}-\frac{1}{70}=\frac{6}{70}=\frac{3}{35}\)
=>A=\(\frac{3}{35}\cdot7=\frac{3}{5}\)
= 7.( \(\frac{1}{10.11}\)+\(\frac{1}{11.12}\)+ ............+\(\frac{1}{69.70}\))
=7.(\(\frac{1}{10}\)- \(\frac{1}{11}\)+ \(\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\))
=7.\(\left(\frac{1}{10}+\left(-\frac{1}{11}\right)+\frac{1}{11}+\left(-\frac{1}{12}\right)+\frac{1}{12}+...+\frac{1}{69}+\left(-\frac{1}{70}\right)\right)\)
=7.\(\left(\frac{1}{10}+\left(-\frac{1}{70}\right)\right)\)
=7.\(\frac{6}{70}\)
=\(\frac{42}{70}\)
S=7/10.11+7/11.12+...+7/69.70
=>S= 7.(1/10.11+1/11.12+...+1/69.70)
=> S= 7. ( 1/10-1/11+...+1/69-1/70)
=> S= 7. ( 1/10 - 1/70)
=> S = 7. 3/35
=> S = 3/5