Bài giải

$\left( 1+\frac{1}{2} \right)\times \left( 1+\frac{1}{3} \right)\times \left( 1+\frac{1}{4} \right)\times ...\times \left( 1+\frac{1}{49} \right)-\left( 1-\frac{1}{2} \right)\times \left( 1-\frac{1}{3} \right)\times \left( 1-\frac{1}{4} \right)\times \left( 1-\frac{1}{5} \right)$

= $\frac{3}{2}\times \frac{4}{3}\times \frac{5}{4}\times ...\times \frac{50}{49}-\frac{1}{2}\times \frac{2}{3}\times \frac{3}{4}\times \frac{4}{5}$

= $\frac{50}{2}-\frac{1}{5}=\frac{250}{10}-\frac{2}{10}=\frac{248}{10}=\frac{124}{5}$