Bài giải:

$\begin{align} & \left( 1-\frac{1}{4} \right)\times \left( 1-\frac{1}{9} \right)\times \left( 1-\frac{1}{16} \right)\times ...\times \left( 1-\frac{1}{100} \right)\times \left( 1-\frac{1}{121} \right) \\ & =\frac{3}{4}\times \frac{8}{9}\times \frac{15}{16}\times ...\times \frac{99}{100}\times \frac{120}{121} \\& =\frac{1\times 3}{2\times 2}\times \frac{2\times 4}{3\times 3}\times \frac{3\times 5}{4\times 4}\times ...\times \frac{9\times 11}{10\times 10}\times \frac{10\times 12}{11\times 11} \\ & =\frac{1}{2}\times \frac{12}{11}=\frac{6}{11} \\\end{align}$