(1) digits are "1".

Quick formula: the head is multiplied by the head, the head is added with the head, and the tail is 1 (if the head and the head are greater than 10, carry it).

(2) The decimal number is "1".

Fast formula: head 1, tail plus as, tail by tail (over 10, carry).

(3) Every single number is "9".

Quick formula: Add 1 to each head, multiply it by 10, subtract the added number, and finally put 1.

(4) All ten digits are 9.

Quick formula: 100 minus the first number and then minus the last number. 100 MINUS everyone, the results are multiplied by each other, accounting for 2 places.

(5) The same head and complementary tail. (mantissa complementation: mantissas add up to 10).

The formula for quick calculation: add 1 to the head joint, and the tail-to-tail accounts for 2 digits.

(6) The heads are complementary and the tails are the same.

Quick formula: head multiplied by head plus tail, tail multiplied by tail, accounting for 2 digits.

(7) Multiply the complement by the overlapping number.

Quick formula: add 1 to the head, then multiply it by the head, and multiply it by the tail, taking 2 digits.

(8) One of them is 1 1.

Quick formula: the beginning and the end are fixed, and the middle is added.