# Interesting Summation

Algebra Level 4

The summation of

m + mm + mmm +mmmm ............up to n terms

Can be represented by $$\frac{m}{a}\left [ 10^{n+1}-10-9n \right ]$$

Find the value of a.

Note
1) m is integer between 1 to 9
2) mm is not m×m. mm is a number.

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