... 2 012 7a (1, 0) Tớnh tng : S = 12 C2 012 + 22 C2 012 + 32 C2 012 + + 2 012 2 C2 012 k k k k k 2C2 012 = k ( k 1) + C2 012 = k ( k 1) C2 012 + kC2 012 k = 1, 2, , 2 012 k k 2C2 012 = k ( k 1) 0,25 2 012 ! ... 2 012 ! k k +k = 2 012 (2 011 C2 010 + C2 011 )k = 1, , 2 012 0,25 k !( 2 012 k ) ! k !( 2 012 k ) ! 2 010 2 011 T ú S = 2 012 2 011 ( C2 010 + C2 010 + + C2 010 ) + ( C2 011 + C2 011 + + C2 011 ) 0,25 2 011 (1 ... A C A (loại) 7b (1, 0) 6 x= y= B ; ,C ; 5 5 C2 012 C2 012 C2 012 C 2 012 + + + + 2 012 2 013 2 012 ! k C2 012 k !( 2 012 k ) ! 2 013 ! k +1 = = = C2 013 k +1 k +1 2 013 ( k + 1) ! 2 013 ...