... C 20 12 +2 C 2 20 12 +3 C 20 12 20 12 + L + 20 122 C20 12 k k k k k 2C20 12 = k ( k 1) + C20 12 = k ( k 1) C20 12 + kC2012k = 1, 2, , 20 12 20 12! 20 12! k k k k 2C20 12 = k ( k 1) +k = 20 12( 2011C2010 ... C2011 )k = 1, , 20 12 k !( 20 12 k ) ! k !( 20 12 k ) ! 20 10 20 11 T ú S = 20 12 20 11( C2010 + C2010 + L + C2010 ) + ( C2011 + C2011 + L + C2011 ) = 20 12 20 11( + 1) 20 10 + ( + 1) 20 11 = 20 12 ... C C C C 20 12 Cõu 7b(1,0 im) Tớnh tng : T = 20 12 + 20 12 + 20 12 + L + 20 12 2013 20 12! k C20 12 k !( 20 12 k ) ! 20 13! k +1 = = ì = ìC2013 k +1 k +1 20 13 ( k + 1) ! 20 13 ( k + 1) ! 20 13 T =...