\(a=\frac{1}{1\times2}+\frac{1}{2\times3}+...+\frac{1}{99\times100}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)

\(=1-\frac{1}{100}=\frac{99}{100}\)

cảm ơn!!!

Chứng minh chứ gì?

abcdeg=10000.ab+100.cd+eg=9999.ab+99.cd+(ab+cd+eg) chia hết cho 11