... + +n 1 n 1 0 n 1 0 0 1 n 1 1 1 n 1 n 1 n n nC a b C a b C a b+ − − + − ++ + + +n n 1 n n 0 n 0 0 1 1 n 1 1 1 n n nC a b C a b C a b( )− −− − + − ++ + +n n 1 n 1 n 1 1 n n n n 1 n n ... + = 1 2 n n 1 b b b b( ) ( )+ += + + + − ⋅ − + ⋅ 1 2 n n 1 n n 1 b b 1 b b 1 b b+≥ + + + ⋅ + = + 1 2 n n 1 b b b b 1 n 1 ,nghóa là+ ++ ++ + +⋅ ⋅ ⋅ ⋅ ⋅ ⋅ 1 2n 1 n 1 1 2 n 1 1 2 n 1 a ... ⋅22n 1 1 2 n 1 aba a a, ++++=⋅ ⋅ ⋅n 1 n 1 n 1 1 2 n 1 aba a a,ta được ( )− +⋅ ⋅ ⋅ ⋅ ⋅ = 1 2 n 1 n n 1 b b b b b 1 và do giả thuyết quy nạp,− ++ + + + ⋅ ≥ 1 2 n 1 n n 1 b b...