Find the sum of nnn terms of the sequence 7,77,777,…7, 77, 777, \dots7,77,777,….
781(10n+1−10−9n)\frac{7}{81}(10^{n+1} - 10 - 9n)817(10n+1−10−9n)
79(10n−1)\frac{7}{9}(10^n - 1)97(10n−1)
781(10n−9n)\frac{7}{81}(10^n - 9n)817(10n−9n)
7n(n+1)/27n(n+1)/27n(n+1)/2