\(\frac{1}{1\times2}+\frac{1}{2\times3}+...+\frac{1}{99\times100}\)
1/1x2 + 1/2x3 + ... + 1/99x100
= 1/1 - 1/2 + 1/2 - 1/3 + ... + 1/99 - 1/100
= 1/1 - 1/100
= 99/100.
\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+........+\frac{1}{99}-\frac{1}{100}\)
=\(1-\frac{1}{100}=\frac{99}{100}\)