Tính : `B = ( 1/2 - 1 ) . ( 1/3 - 1 ) ... ( 1/2008 - 1 ) . ( 1/2009 - 1 )`

$\text{B = ($\frac{1}{2}$ - 1) . ($\frac{1}{3}$ - 1) ... ($\frac{1}{2008}$ - 1) . ($\frac{1}{2009}$ - 1) }$

$\text{= $\frac{1}{2}$ . $\frac{2}{3}$ ... $\frac{2007}{2008}$ . $\frac{2008}{2009}$ }$

$\text{= $\frac{1}{2009}$ }$