Cho tổng trên là A
\(A=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{2003.2005}\)
\(A=2\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2003}-\frac{1}{2005}\right)\)
\(A=2\left(1-\frac{1}{2005}\right)\)
\(A=2.\frac{2004}{2005}\)
\(A=\frac{4008}{2005}\)