$\large \underbrace{111\ 111\ 111^2}_{\text{Number of 1 = 9}} = ?$

The answer is 12345678987654321.

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$1²=1$

$11²=121$

$111²=12321$

$1111²=1234321$

This is a sequence. So our answer will be $111111111²=$ $\boxed{12345678987654321}$