\(S=\frac{1}{u_2-u_1}\left(\frac{1}{u_1}-\frac{1}{u_2}\right)+\frac{1}{u_3-u_2}\left(\frac{1}{u_2}-\frac{1}{u_3}\right)+...+\frac{1}{u_{50}-u_{49}}\left(\frac{1}{u_{49}}-\frac{1}{u_{50}}\right)\)
\(=\frac{1}{d}\left(\frac{1}{u_1}-\frac{1}{u_2}\right)+\frac{1}{d}\left(\frac{1}{u_2}-\frac{1}{u_3}\right)+...+\frac{1}{d}\left(\frac{1}{u_{49}}-\frac{1}{u_{50}}\right)\)
\(=\frac{1}{d}\left(\frac{1}{u_1}-\frac{1}{u_{50}}\right)\)
Mặt khác \(S_{100}=\frac{100\left(2+99d\right)}{2}=24850\Rightarrow d=5\)
\(\Rightarrow u_{50}=u_1+49d=246\)
\(\Rightarrow S=\frac{1}{5}\left(1-\frac{1}{246}\right)=....\)