S = C C 10 20 0 10 + C C 1 10 20 9 + C C 2 10 20 8 +

1.Tớnh tụ̉ng : S = C C ^{10 20 0 10} + C C ^{1 10 20 9} + C C ^{2 10 20 8} + ... + C C ^{10 20 9 1} + C C ^{10 0 10 20}