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If 2 3 1029 + 1 3 1029 + n 6 + 1 \large \frac{ 23^{1029} + 13^{1029} + n }{6 + 1} is an integer, then find the lowest possible value of n ( where n is a whole number ).


The answer is 0.

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2 solutions

We have to work mod 7 , since the denominator is 7. Hence,

2 3 1029 + 1 3 1029 + n 0 23^{1029} + 13^{1029} + n ≡ 0 (mod 7)

Now, 2 3 1029 2 1029 23^{1029} ≡ 2^{1029} (mod 7)

Also, 2 3 1 2^3 ≡ 1 (mod 7) ( 2 3 ) 343 1 343 1 \Rightarrow (2^3)^{343} ≡ 1^{343} ≡ 1 (mod 7)

Along similar lines, 1 3 1029 ( 1 ) 1029 1 13^{1029} ≡ (-1)^{1029} ≡ -1 (mod 7)

Hence, 2 3 1029 + 1 3 1029 + n 0 1 1 + n 0 23^{1029} + 13^{1029} + n ≡ 0 \Leftrightarrow 1 -1 +n ≡ 0 (mod 7)

The lowest possible whole number is 0 \boxed{0}

Abhijeet Verma
Mar 15, 2015

23 = 7 k + 2 23=7k+2 This implies that 23 × 23 × 23 = 7 y + 1 23 \times 23 \times 23=7y+1 ( using modular arithmetic) This implies that 2 3 1029 = 7 m + 1 23^{1029}=7m+1 Similarly, 1 3 1029 = 7 n + 6 13^{1029}=7n+6 Therefore 2 3 1029 + 1 3 1029 = 7 k 23^{1029}+13^{1029}=7k Thus n=0 & n=7

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