Question

The sum of a digit of a number 10n-1 is 3798. The value of n is

Answer 1

$10n = 3798 + 1 = 3799 \\ n = 3799 \div 10 = 379.9$

Answer 2

Given, sum of 10n-1=3798
First term= 10*1-1=10-1=9.
a=9.
Sum of 2 terms =10*2-1=20-1=19.
Second term=Sum of 2 terms-First term=19-9=10.
Sn=3798
2a+(n-1)d=3798
2*9+(n-1)10=3798
18+10n-10=3798
18-10+10n=3798.
8+10n=3798.
10n=3798-8
10n=3790
n=379.

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