1/1.3 + 1/3.5 + 1/5.7+ . ... + 1/2009.2011= 1/2. (2/1.3 + 2/3.5 + 2/5.7 + ... + 2/2009.2011)= 1/2. (1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/2009 - 1/2011)= 1/2. ( 1 - 1/2011) = 1/2. 2010/2011 = 1005/2011
đáp án là B nha
B đúng