Can you divide?

Find the remainder when 3 2 3 2 32 32^{32^{32}} is divided by 7.


The answer is 4.

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2 solutions

Otto Bretscher
Nov 22, 2015

3 2 3 2 32 4 3 2 32 4 ( 1 ) 32 4 ( m o d 7 ) 32^{32^{32}}\equiv 4^{32^{32}} \equiv 4^{(-1)^{32}}\equiv \boxed{4} \pmod7 . Since 4 3 = 64 1 ( m o d 7 ) 4^3=64\equiv 1 \pmod7 , we can work mod 3 in the exponent.

Neelesh Vij
Nov 21, 2015

First of all we need to check out how we can write 3 2 32 32^{32} . it is equal to ( 33 1 ) 32 (33-1)^{32} or in the form of 3 T + 1 3T+1 where T is a natural number now going back to the question 3 2 3 2 32 32^{32^{32}} is equivalent to 3 2 3 T + 1 32^{3T+1} = 2 15 T + 5 2^{15T+5} = 2 15 T + 3 × 2 2 2^{15T+3} \times 2^{2} = ( 7 + 1 ) 15 T + 1 × 4 (7+1)^{15T+1} \times 4 this term on evaluating by binomial expansion we get several terms but the last term will have value 1 1 rest all terms will have at least 1 1 exponent of 7 7 and will be divisible by 7 7 . so our main interest lies in the last term which is not divisible by 7 7 so going back to expression we get (many terms which are divisible)*4 + 1 × 4 1 \times 4 which gives us a remainder of 4 \boxed{4}

It's much simpler if you apply Fermat's Little Theorem .

Pi Han Goh - 5 years, 6 months ago

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Could u please post ur solution

Bala vidyadharan - 5 years, 6 months ago

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3 2 3 2 32 ( 32 m o d 7 ) 3 2 32 4 3 2 32 m o d ϕ ( 7 ) 4 3 2 32 m o d 6 32^{32^{32}} \equiv (32\bmod 7)^{32^{32}} \equiv 4^{32^{32} \bmod \phi(7)} \equiv 4^{32^{32} \bmod 6}

So we want to evaluate 3 2 32 m o d 6 32^{32} \bmod 6 which is equal to ( 36 4 ) 32 4 32 1 6 16 4 16 1 6 8 4 8 1 6 4 4 4 1 6 2 4 2 4 (36-4)^{32} \equiv 4^{32} \equiv 16^{16} \equiv 4^{16} \equiv 16^8 \equiv 4^8 \equiv 16^4 \equiv 4^4 \equiv 16^2 \equiv 4^2 \equiv 4

We are left to evaluate 4 4 m o d 7 4^4 \bmod7 which is equal to 1 6 2 2 2 = 4 16^2 \equiv 2^2 =\boxed4 .

Pi Han Goh - 5 years, 6 months ago

How can I rate my problem?

neelesh vij - 5 years, 6 months ago

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