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Question

Find the sum to n terms (7+77+777+.....n terms)

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Solution

Let s be the sum of n terms of sumes,
(7+77+777+....n terms)
7+77+777+....n terms
=7(1+11+111+....n terms )
=79(9+99+999+....nterms)
=79[(101)+(1021)(1031)+....nterms]
=79[(10+102+103....tontrems)(1+1+...nterms)]
=79[10(10n1)101n]
=79[109(10n1)n]

1208462_1347065_ans_5a0960e81dfc4496b2c438e1c48c2db9.jpg

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