Ta có :

A = \(\frac{1}{4.9}+\frac{1}{9.14}+\frac{1}{14.19}+...+\frac{1}{64.69}\)

5A = \(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+...+\frac{5}{64.69}\)

5A = \(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{64}-\frac{1}{69}\)

5A = \(\frac{1}{4}-\frac{1}{69}\)

5A = \(\frac{65}{276}\)

A = \(\frac{65}{276}:5\)

A = \(\frac{13}{276}\)

13 /276

thiếu

\(A=1.2.3...9-1.2.3..8-1.2.3....8.8\)

\(A=1.2.3...8.\left(9-1-8\right)\)

\(A=0.1.2.3.....8=0\)

\(B=\left(3.4.2^{16}\right)^2=\left(3.2^2.2^{16}\right)^2=\left(3.2^{18}\right)^2=9.2^{36}\)

1/4*9+1/9*14+1/14*19+...+1/1999*2004

= 1/5 (1/4 - 1/9 + 1/9 - 1/14 + ... + 1/1999 - 1/2004)

= 1/5 (1/4 - 1/2004)

= 1/5 (501/2004 - 1/2004)

= 1/5 . 125/501

= 25/501

\(\frac{1}{4\times9}+\frac{1}{9\times14}+\frac{1}{14\times19}+...+\frac{1}{1999\times2004}\)

\(=\frac{1}{5}\times\left(\frac{5}{4\times9}+\frac{5}{9\times14}+\frac{5}{14\times19}+...+\frac{5}{1999\times2004}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{1999}-\frac{1}{2004}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{4}-\frac{1}{2004}\right)\)

\(=\frac{1}{5}\times\frac{500}{2004}=\frac{25}{501}\)

khó nhìn quá bn ơi

bạn đọc và viết ra giấy sẽ hiểu mà oki

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