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25 October, 21:09

Sum to n terms the series 7 + 77 + 777

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  1. 25 October, 21:11
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    Step-by-step explanation:

    =7 (1+11+111+1111 ... n)

    =7/9 (9+99+999+9999 ... n)

    =7/9 ((10-1) + (10^2-1) + (10^3-1) + ... n)

    =7/9 ((10+10^2+10^3 ... n) - (1+1+1+1 ... n))

    =7/9 ((10 (10^n-1) / (10-1)) - n)
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