A: Then the positive total tile number of the nth number is (n+ 1)2.
(2) The number of black tiles in the nth graph can be expressed as (1+2+3+…+n).
When n=20,1+2+3+…+20 = 210 (block),
Answer: The number of black tiles in the 20th picture is 2 10.
(3) According to the above reasoning, the number of white tiles in the nth figure can be expressed as: (n+1) 2-(1+2+3+...+n) = (n+1) × (n+2) ÷ 2.
(n+ 1)×(n+2)÷2，
=(55+ 1)×(55+2)÷2,
=56×57÷2,
= 1596 (block),
Answer: The number of white tiles in the nth figure can be expressed by the formula: (n+ 1)(n+2)÷2. It is calculated that there are 1596 white tiles in the 55th figure.
So the answer is: (1)(n+ 1)2 pieces.