1/1×5 + 1/5×9 + 1/9×13 +.+1/2005×2009
= (1/4) × (1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + ...+ 1/2005 - 1/2009)
= (1/4) × (1 - 1/2009)
= 502/2009
1/1×5 + 1/5×9 + 1/9×13 +.+1/2005×2009
= (1/4) × (1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + ...+ 1/2005 - 1/2009)
= (1/4) × (1 - 1/2009)
= 502/2009