... 20 09 1 1 1 1 1 1a a 1 2 2 3 20 09 20101120100,25Mặt khác: ( ) + = ữ = = >22008 1 2008 20 09 2010 20 09 20101201020 09 2010 20 09 20 09 12010 2 20 09 02010 20 09 2010 20 09 0,25nên ... +10=0,253)1,0điểm1 220 09 1 2008 1C = ()()1 2 1 21 220 09 1 2008 1 20 09 1 2008 120 09 1 2008 1 + = + ()()2 21 21 220 09 1 2008 120 09 1 2008 1 = + 0,252 21 220 09 1 2008 120 09 1 2008 ... + ( ) ( )2 220 09 2008 20 09 200820 09 1 2008 1 += + 2 2401720 09 1 2008 1= + 0,25 Mà 4017 4018 2.20 09 < = 2 2401720 09 1 2008 1 + < 2 2401820 09 1 2008 1 + 0,25...