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# If p is a prime number greater than 3, find the remainder when p^2 + 1

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Joined: 02 Sep 2009
Posts: 62353
If p is a prime number greater than 3, find the remainder when p^2 + 1  [#permalink]

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06 Feb 2020, 08:23
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81% (00:56) correct 19% (01:36) wrong based on 54 sessions

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If p is a prime number greater than 3, find the remainder when p^2 + 17 is divided by 12.

A. 8
B. 7
C. 6
D. 1
E. 0

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If p is a prime number greater than 3, find the remainder when p^2 + 1  [#permalink]

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06 Feb 2020, 08:45
1
Bunuel wrote:
If p is a prime number greater than 3, find the remainder when p^2 + 17 is divided by 12.

A. 8
B. 7
C. 6
D. 1
E. 0

Test any number. Let p = 5 for example. $$p^2 + 17 = 42$$. 42 divided by 12 has a remainder of 6.

Ans: C

For the proof of why the remainder is always 6:

Let n be any prime number greater than 3. We can write n as $$n = E+ 1$$ where E is even.

Then $$n^2 = (E + 1)^2 = E^2 + 2E + 1$$.

The claim is $$E^2 + 2E$$ will always have a factor of 12. We need to prove it has a factor of both 4 and 3.

The factor of 4 is easier to prove. Since $$E^2 + 2E = E*(2+E)$$, both E and (2+E) have a factor of 2 so the product must have a factor of 4.

For the factor of 3, recall E + 1 is a prime number greater than 3 so it cannot be divisible by 3. Thus either E or E + 2 must be divisible by 3. Both are factors of the product, therefore in any case $$E^2 + 2E$$ must be divisible by 3.

Finally, we proved $$E^2 + 2E$$ is divisible by 12, then $$n^2 = E^2 + 2E + 1$$ divided by 12 will have a remainder of 1 and $$n^2 + 17$$ must have a remainder of 6.
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Re: If p is a prime number greater than 3, find the remainder when p^2 + 1  [#permalink]

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08 Feb 2020, 09:17
P^2+17/12, Remainder R=?
P^2+17=12a+R
P^2+17-12a=R
P is a prime number and greater than 3 and 12a could be any multiple of 12

Let's use a trial and error method
If P=5 and a=1,2,3
25+17-12=30 (Substituting a = 1)
25+17-24=18 (Substituting a = 2)
25+17-36=6 (Substituting a = 3)

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Re: If p is a prime number greater than 3, find the remainder when p^2 + 1  [#permalink]

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08 Feb 2020, 12:30
Bunuel wrote:
If p is a prime number greater than 3, find the remainder when p^2 + 17 is divided by 12.

A. 8
B. 7
C. 6
D. 1
E. 0

We see that p can be any prime number greater than 3, so it could be 5, 7, 11, 13, etc. Let’s choose the smallest possible value for p.

If p = 5, we have:

25 + 17 = 42

42/12 = 3 remainder 6

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Re: If p is a prime number greater than 3, find the remainder when p^2 + 1  [#permalink]

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09 Feb 2020, 02:15
Prime number greater than 3 = 5, 7, 11, 13………..
Suppose, P= 5,
P^2 + 17 = 5^2 + 17 = 25 + 17 = 42
Now 42/ 12 = 3 + 6/12, Remainder = 6(C)
Re: If p is a prime number greater than 3, find the remainder when p^2 + 1   [#permalink] 09 Feb 2020, 02:15
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