\(\frac{1}{2003.2002}-\frac{1}{2002.2001}-...-\frac{1}{2.1}\)
\(=\frac{1}{2003.2002}-\left(\frac{1}{2002.2001}+\frac{1}{2001.2000}+...+\frac{1}{2.1}\right)\)
\(=\frac{1}{2003.2002}-\left(\frac{1}{2002}-\frac{1}{2001}+\frac{1}{2001}-\frac{1}{2000}+...+\frac{1}{2}-1\right)\)
\(=\frac{1}{2003.2002}-\left(\frac{1}{2002}-1\right)\)
\(=\frac{1}{2003.2002}-\left(\frac{1}{2002}-\frac{2002}{2002}\right)\)
\(=\frac{1}{2003.2002}-\frac{-2001}{2002}\)
\(=\frac{1}{2003}-\frac{1}{2002}+\frac{2001}{2002}\)
\(=\frac{1}{2003}+\frac{2000}{2002}\)
\(=\frac{1}{2003}+\frac{1000}{1001}\)
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