9. Here are some numbers where all the digits are 9: 9, 99, 999, 9999, 99999, … In numbers where all the digits are 9, which are multiples of 11?

Translation of the question: Вот числа, в которых все цифры — 9: 9, 99, 999, 9999, 99999, … Какие из чисел, состоящих только из цифр 9, кратны 11?

For divisibility by 11, the difference between the sums of alternating digits must be divisible by 11. In a number made entirely of 9s, this difference is 0 when the number of digits is even, and 9 when it is odd. Therefore, numbers with an even number of digits are divisible by 11.

Answer (English): 99, 9999, 999999, … — all such numbers with an even number of digits.

Перевод ответа: 99, 9999, 999999, … — все такие числа, в которых чётное количество цифр.