If p and q are prime numbers which satisfy the equation
p + q = ( p − q ) 3 ,
what is the value of p − q + 2 p q ?
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nice :) But may wanna try in Latex for future solutions
Start from 2,3,5,7..Their cubes 8,27,125,..So least value is 8 hence 5,3 satisfies.
Nice..by the way p-q=1 is possible for the primes 3 and 2.
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1^3=1but addition of any two no must be greater than1 so it is not satisfying the given eq.
is thz ok
what was the logic.....i mean how did u think.....can u please elaborate!!
you are 32
@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 ?
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(p - q) also divides (q - p); therefore, it divides (p + q) + (q - p) = 2q.
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.
Here's a solution that I don't think anybody has mentioned yet.
Suppose that p , q > 3 . Note that all primes p greater than 3 satisfy p ≡ ± 1 ( m o d 6 ) . This means that the residues of p and q are either equal to each other or negatives of each other. But then exactly one of p + q or p − q is divisible by six, contradiction! (For example, if p ≡ q ≡ 1 ( m o d 6 ) , then p − q ≡ 0 ( m o d 6 ) while p + q ≡ 2 ( m o d 6 ) . But it must hold true that p + q ≡ ( p − q ) 3 ≡ 0 2 ≡ 0 ( m o d 6 ) , which doesn't hold.) Therefore one of p or q must be two or three. After some experimentation, we find ( p , q ) = ( 5 , 3 ) does indeed work, and the requested expression has value 5 − 3 + 2 ⋅ 5 ⋅ 3 = 3 2 .
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
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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
Post it as solution not comment
p=5, q=3.
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
(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
fantastic
Why (p-q)^3=p+q <=> 2q ?
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
Easily check for the smallest cube after 1 that is 8 . It can be written as 5 + 3 where 5 and 3 are prime. Now 5 + 3 = ( 5 − 3 ) 3 = 8 . So, p − q + 2 p q = 5 − 3 + 2 × 5 × 3 = 3 2 .
(5+3)=8 and (5-3)^3=8 then p=5 , Q=3; so 5-3+2 * 5 * 3=32
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?
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....
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.
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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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