Prime

Find a prime number p p such that ( p 1 ) 3 = 134 p 14 \large (p - 1)^3 =134p - 14


The answer is 13.

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3 solutions

Akeyla Naufal
Jun 2, 2017

( p 1 ) 3 1 ( m o d p ) (p - 1)^3 \equiv -1 \pmod p while 134 p 14 14 ( m o d p ) 134p - 14 \equiv -14 \pmod p . Since LHS equal to RHS , 1 14 ( m o d p ) -1 \equiv -14 \pmod p . So , 13 13 is divisible by p p . The only possible value for p p is 13 13 . Check p = 13 p = 13 onto both sides yields equality . Therefore , the solution is p = 13 p = 13

@Syed Hissaan , since (p-1) = -1 (mod p) , (p-1)^3 = (-1)^3 (mod p) = -1 (mod p).

Akeyla Naufal - 4 years ago

how is (p-1)^3 = -1(mod p) ?

Syed Hissaan - 4 years ago

Nice, quick use of modular arithmetic instead of expanding and solving algebraically!

Zach Abueg - 3 years, 12 months ago
Sam Dave
Jun 4, 2017

rearrange the equation by taking 2 as common in RHS,now given that p is prime and whole cube of LHS is equal to RHS so whole cube of (p-1) must have two as one of its factor,now in RHS by using congruences we can conclude that 3p-7 is congruent to 0 mod 4(as 2(67p-7) must be divisible by 8 so 67p-7 must be divisible by 4) so we can write 3p-4r=7,its a linear diophantine eqn (r is an integer).on solving p=5+4k(k is an integer) and k = 2 gives p=13.

Scrub Lord
Jun 3, 2017

p^3 - 3p^2 + 3p - 1 = 134p - 14

p^3 - 3p^2 - 131p = -13

p(p^2 -3p -131) = -13

p has to be prime, so the only possible solution is 13.

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