Sum 0.9 + 0.99 + 0.999 + 0.9999 + +0.99999…… 9 (n 9S) this is in the sequence

0.9+0.99+0.999+0.9999+…… +0.99999…… nine
=1-0.1+1-0.01+…… +1-0.00000000.1
=n-(1-0.1^n)/9

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