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# I.S.I. 2015 Subjective 2 Solution

This is I.S.I. 2015 Subjective 2 Solution (from ISI Entrance). The book, Test of Mathematics at 10+2 Level is Published by East West Press. This problem book is indispensable for the preparation of I.S.I. B.Stat and B.Math Entrance.

Also visit: I.S.I. & C.M.I. Entrance Course of Cheenta

## Problem

Find all such Natural number n such that 7 divides $5^n + 1$

## Discussion:

Teacher: This is simple case of modular arithmetic. Consider the set of all residues of $5^n$ modulo 7 and you will see a pattern

Student: Sure. $5^1 = 5 , 5^2 = -3, 5^3 = -1 , 5^4 = 2 , 5^5 = 3 , 5^6 = 1$ modulo 7. The residues repeat after that since $5^{6k + m} = 5^{6k} \times 5^m = 5^m$ modulo 7 where $m \le 6$ .

We want $5^n = -1$ mod 7 hence n must be an odd multiple of 3 or n = 6r + 3 for any nonnegative integer r.