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Find the remainder when 3 2002 + 7 2002 + 2002 3^{2002} + 7^{2002} + 2002 is divided by 29.

1 2 0 3

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2 solutions

Ankit Kumar Jain
Feb 29, 2016

3 2002 = 9 1001 3^{2002} = 9^{1001} and 7 2002 = 4 9 1001 7^{2002} = 49^{1001} .

a n + b n = ( a + b ) ( a n 1 a n 2 × b + a n 3 × b 2 b n 1 ) a^{n} + b^{n} = (a + b)(a^{n-1} - a^{n-2}\times{b} + a^{n-3}\times{b^{2}} - \cdot \cdot \cdot - b^{n-1}) where n n is odd.

( a + b ) ( a n + b n \therefore (a + b) \mid (a^{n} + b^{n} ) , where n n is odd.

( 9 + 49 = 58 ) 9 1001 + 4 9 1001 29 9 1001 + 4 9 1001 \therefore (9 + 49 = 58) \mid 9^{1001} + 49^{1001} \Rightarrow 29 \mid 9^{1001} + 49^{1001} .

3 2002 + 7 2002 + 2002 2002 1 ( m o d 29 ) \Rightarrow 3^{2002} + 7^{2002} + 2002\equiv 2002\equiv1\pmod{29}

Moderator note:

Good observation of the special values that were selected in this problem.

How could we deal with the more general case?

Exactly Same Way.

Kushagra Sahni - 5 years, 3 months ago

@Calvin Lin . For a general case I would go with Akshat Sharda's solution...

Ankit Kumar Jain - 5 years, 3 months ago

@Ankit Kumar Jain nicely done. I didn't think of that :)

Ankit Nigam - 5 years, 3 months ago

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Thanks.. BTW a good question..Where did you get the question from??

Ankit Kumar Jain - 5 years, 3 months ago

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Well, i have a book for preparation of ISI(Indian Statistical Institute) entrance exam. This question is from that book. :)

Ankit Nigam - 5 years, 3 months ago
Akshat Sharda
Feb 29, 2016

3 2002 + 7 2003 + 2002 ( m o d 29 ) 3 71 ϕ ( 29 ) 3 14 + 7 71 ϕ ( 29 ) 7 14 + 29 ( 69 ) + 1 ( m o d 29 ) 3 14 + 7 14 + 1 ( 3 3 ) 4 3 2 + ( 7 3 ) 4 7 2 + 1 ( m o d 29 ) ( 27 ) 4 9 + ( 343 ) 4 49 + 1 ( 2 ) 4 9 + ( 5 ) 4 20 + 1 ( m o d 29 ) 16 9 + ( 25 ) 2 20 + 1 144 + ( 4 ) 2 20 + 1 ( m o d 29 ) 28 + 320 + 1 = 349 1 ( m o d 29 ) \begin{aligned} 3^{2002}+7^{2003}+2002 & \pmod {29} \\ 3^{71\phi(29)}\cdot 3^{14}+7^{ 71\phi(29)}\cdot 7^{14}+29(69)+1 & \pmod {29} \\ 3^{14}+7^{14}+1 \equiv (3^{3})^4 3^2+(7^{3})^4 7^2+1 & \pmod {29} \\ (27)^4 9+(343)^4 49+1 \equiv (-2)^4 9+(-5)^4 20+1 & \pmod {29} \\ 16\cdot 9 + (25)^2 20+1 \equiv 144+(-4)^2 20+1 & \pmod {29} \\ 28+320+1 = 349 \equiv \boxed{1} & \pmod {29} \end{aligned}

@Akshat Sharda What is the latex code for line break i.e if I want too start off with a new line..??

Ankit Kumar Jain - 5 years, 3 months ago

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Toggle latex of my solution.

Akshat Sharda - 5 years, 3 months ago

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How to do that??? Actually I am not so familiar with all this..

Ankit Kumar Jain - 5 years, 3 months ago

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@Ankit Kumar Jain You have opened brilliant but in mobile version or full site ?

Akshat Sharda - 5 years, 3 months ago

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@Akshat Sharda Is it use of double slash??

Ankit Kumar Jain - 5 years, 3 months ago

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@Ankit Kumar Jain Yes ... You are right

Akshat Sharda - 5 years, 3 months ago

@Akshat Sharda Oh I got it . Thanks Akshat But what is the use of \begin{equation}\begin{split}??

Ankit Kumar Jain - 5 years, 3 months ago

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@Ankit Kumar Jain This is used to align all the (mod 29) in a straight line to make solution look beautiful.

Akshat Sharda - 5 years, 3 months ago

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