\(\frac{1}{100.99}-\left(\frac{1}{99.98}+\frac{1}{98.97}+...+\frac{1}{2.1}\right)\)
\(=\frac{1}{100}-\frac{1}{99}-\left(\frac{1}{99}-\frac{1}{98}+\frac{1}{98}-\frac{1}{97}+...+\frac{1}{2}-1\right)\)
\(=\frac{1}{100}-\frac{1}{99}-\left(\frac{1}{99}-1\right)\)
\(=\frac{1}{100}-\frac{1}{99}-\frac{1}{99}+1\)
\(=\frac{9799}{9900}\)