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This is a problem where I like to find a pattern before formalizing.
Note that 1 0 9 9 ÷ 1 0 1 0 0 = 1 0 − 1 .
Then, 1 0 9 8 ÷ 1 0 − 1 = 1 0 9 9 .
Then, 1 0 9 7 ÷ 1 0 9 9 = 1 0 − 2 .
Then, 1 0 9 6 ÷ 1 0 − 2 = 1 0 9 8 , and so on.
Continuing this pattern, we will get 1 0 − 5 0 for the entire result.
To formalize the solution, we can set a 1 = 1 0 9 9 ÷ 1 0 1 0 0 = 1 0 − 1 and a k = 1 0 1 0 0 − k ÷ a k − 1 for k = 2 , 3 , … , 9 9 . Using induction , we can prove the pattern we saw above.