2 Prime Numbers Problem

If p p and q q are prime numbers which satisfy the equation

p + q = ( p q ) 3 , p+q = (p-q)^3,

what is the value of p q + 2 p q p - q + 2pq ?

8 17 32 26

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13 solutions

Richard Rodriguez
Apr 16, 2014

p+q= (p-q)^2 * (p-q), then, (p - q) divides (p + q). This implies that (p - q) divides 2q.

If (p - q) divides q, then (p - q) = 1 or (p - q) = q, which are impossible statements. Now, notice that (p - q) divides 2. As (p - q)=1 is impossible for prime numbers, then (p - q) = 2.

From the original equality, we have p + q = 8, then the only prime numbers that satisfy this are 5 and 3.

From (p - q)^3 >0 we know that p>q, then p = 5 q = 3

Thus p - q + 2pq = 5 - 3 + 2(5)(3) = 32

nice :) But may wanna try in Latex for future solutions

Happy Melodies - 7 years, 1 month ago

Start from 2,3,5,7..Their cubes 8,27,125,..So least value is 8 hence 5,3 satisfies.

BHinstitute Harendran B - 7 years, 1 month ago

Nice..by the way p-q=1 is possible for the primes 3 and 2.

Vijaysekhar Chellaboina - 7 years, 1 month ago

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1^3=1but addition of any two no must be greater than1 so it is not satisfying the given eq.

Ranesh Sarkar - 7 years, 1 month ago

is thz ok

Rohit Singh - 7 years, 1 month ago

what was the logic.....i mean how did u think.....can u please elaborate!!

Max B - 7 years, 1 month ago

you are 32

Pratap Singh - 7 years, 1 month ago

@Richard Rodriguez , 15, Nicaragua can u plz tell ... if then, (p - q) divides (p + q) then how it implies that (p - q) divides 2q. explain ?

taranjot singh - 7 years, 1 month ago

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(p - q) also divides (q - p); therefore, it divides (p + q) + (q - p) = 2q.

Ricardo Moritz Cavalcanti - 5 months, 2 weeks ago

Why is it that (p-q) = 1 is impossible for p and q being prime numbers ? Take p = 3 and q = 2 , they are both prime numbers, and p-q = 1.

Yacine Zahidi - 7 years, 1 month ago
David Altizio
Apr 18, 2014

Here's a solution that I don't think anybody has mentioned yet.

Suppose that p , q > 3 p,q>3 . Note that all primes p p greater than 3 3 satisfy p ± 1 ( m o d 6 ) p\equiv \pm 1\pmod 6 . This means that the residues of p p and q q are either equal to each other or negatives of each other. But then exactly one of p + q p+q or p q p-q is divisible by six, contradiction! (For example, if p q 1 ( m o d 6 ) p\equiv q\equiv 1\pmod 6 , then p q 0 ( m o d 6 ) p-q\equiv 0\pmod 6 while p + q 2 ( m o d 6 ) p+q\equiv 2\pmod 6 . But it must hold true that p + q ( p q ) 3 0 2 0 ( m o d 6 ) p+q\equiv (p-q)^3\equiv 0^2\equiv 0\pmod 6 , which doesn't hold.) Therefore one of p p or q q must be two or three. After some experimentation, we find ( p , q ) = ( 5 , 3 ) (p,q)=(5,3) does indeed work, and the requested expression has value 5 3 + 2 5 3 = 32 5-3+2\cdot 5\cdot 3=\boxed{32} .

the prime no.s are 5 and 3. LHS: 5+3=8. RHS: (5-3)^3=(2)^3=8. so p-q+2pq=5-3+30=32

This is a lengthy method but interesting route to the solution relying on a knowledge of given choices..........Step1 ......Assuming that p and q are unique......only one even prime exists.....so if neither p nor q can be proved to be not 2 .......we can make sure that both p+q and p-q are even..........

step2...... if p=2 or q=2 the solution to p-q+2pq becomes odd.....the only option in the choice which is odd is 17............. but assuming either p or q=2 and back substituting that value alone leads to a decimal value....for the other .......therefore we can confirm that neither value can become 2.........Reduction ad absurdum............

step 3........now both sum and difference of p and q have to be even since only odd prime numbers are involved............ Let p+q=2a & p-q=2b
substituting in p+q=p-q^3 we get a=4(b^3)....... EQ1

now square both p+q=2a and p-q=2b and subtract.......we get 4(a^2)-4(b^2)=4pq

substituting 2pq=2a^2-2b^2 and EQ1 in our question p-q+2pq we get 2b+2a^2-2b^2= 32b^6-2b^2+2b the only value which satisfies the given range of choices is b=1.....putting b=1 gives us the result as 32

Abhay Liginlal - 7 years, 1 month ago

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Another solution:

From ur solution it can be seen that p and q cannot be equal to 2.

The possible value of p+q: 8,27,64,125, etc. //(p-q)power3

p-q is even. So their cube is also even.p+q:8,64,216,etc.

if the sum of two number is 8,64,216,etc, the minimum value of p-q+2pq is 8,64,216,etc.

Given all options are less than 64..so sum must be equal to 8. Now from simple counting,it can be shown that the numbers are 5 and 3 and the correct solution is 32

sanjiv chaudhary - 7 years, 1 month ago

Post it as solution not comment

sanjiv chaudhary - 7 years, 1 month ago

p=5, q=3.

Michel David - 7 years, 1 month ago

let (p-q)=n then we have p-q=n and p+q=n^3 therefore we have ; p=n(n^2+1)/2 and q= n(n^2-1)/2; since p and q have to be prime they should get divided only by themselves or 1 therefore this leaves us with the choice that n has to be 2. Therefore p=5 and q=3. and so answer is 32.

Beautiful

Vijaysekhar Chellaboina - 7 years, 1 month ago

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thx bro

Satya Sainadh Undurti - 7 years, 1 month ago

(p-q)^3=p+q <=> 2q=(p-q-1)(p-q)(p-q+1) ;p>q>=2 SO p-q-1=1 and p-q=2 and p-q+1=q =>p=5,q=3 =>p-q+2pq=32

nice

Vijaysekhar Chellaboina - 7 years, 1 month ago

fantastic

Saiful Islam Selim - 7 years, 1 month ago

Why (p-q)^3=p+q <=> 2q ?

Maxis Jaisi - 7 years ago
Sanjiv Chaudhary
Apr 19, 2014

it can be easily proved that p or q cannot be equal to 2.

The possible value of p+q: 8,27,64,125, etc. //(p-q)power3

p-q is even. So their cube is also even.p+q:8,64,216,etc.

if the sum of two number is 8,64,216,etc, the minimum value of p-q+2pq is 8,64,216,etc.(simple maths,its same as sum only for this expressionexpression)

Given all options are less than 64..so sum must be equal to 8. Now from simple counting,it can be shown that the numbers are 5 and 3 and the correct solution is 32

Ashish Menon
May 28, 2016

Easily check for the smallest cube after 1 1 that is 8 8 . It can be written as 5 + 3 5 + 3 where 5 5 and 3 3 are prime. Now 5 + 3 = ( 5 3 ) 3 = 8 5 + 3 = {(5 - 3)}^3 = 8 . So, p q + 2 p q = 5 3 + 2 × 5 × 3 = 32 p - q + 2pq = 5 - 3 + 2×5×3 = \color{#69047E}{\boxed{32}} .

Mahabubur Rahman
May 2, 2014

(5+3)=8 and (5-3)^3=8 then p=5 , Q=3; so 5-3+2 * 5 * 3=32

Vishnudatt Gupta
Apr 28, 2014

p-q divides p+q likewise If (p - q) divides q, then (p - q) = 1 or (p - q) = q, which are impossible statements. Now, notice that (p - q) divides 2. As (p - q)=1 is impossible for prime numbers, then (p - q) = 2 so we will get p=5 q=3 by solving it we get 32 as answer .

Why (p-q) divides q?

Maxis Jaisi - 7 years ago

Hello,

stated that p and q are prime numbers = [ 1,2,3,5,7,11,......]

p + q = (p - q)^3

(p + q)^(1/3) = (p-q)

p > q , i picked p = 5 , q = 3

so (5 + 3 )^(1/3) = ( 5 - 3 ) = 2,

therefore p - q + 2pq = 5 - 3 + 2(5)(3) = 32...

thanks ah....

Note that 1 is not a prime number.

Calvin Lin Staff - 7 years, 1 month ago
Mohit Priyadarsi
Apr 18, 2014

since p+q/p-q=(p-q)^2 by putting p=3&q=2 then, LHS is not equal to RHS then we go for next two prime number i.e.5 and 3 and we get LHS= RHS.By putting this we get 32 as required answer.

Marvin Oniel
Apr 17, 2014

Let the prime numbers be 5 and 3 for p and q respectively, we know that p>q since (p-q)>0, for the first equation we have 5+3=(5-3)^3=8. Thus, for the second equation we'll have 5-3+2 5 3=32. Which is the right answer.

first you have to think what is the value of (p-q)^3. if p-q=1 then (p-q)^3=1. it doesn't satisfy any p,q prime value. if p-q=2, then (p-q)^3=8. on left side p=5,q=3, so p+q=8, right side, (5-3)^3=8. So p=5, q=3 are the solutions. now just put them on the equation to determine the desired value.

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