Find the value of x when x , y , and z are positive integers such that x ≤ y ≤ z and
x 3 ( y 3 + z 3 ) = 2 0 1 2 ( x y z + 2 )
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
FYI You can format your solution by
This will make it easier for others to read and understand what you are saying.
Log in to reply
thank you sir for your advice. I will surely follow these steps.
x 3 ( y 3 + z 3 ) = 2012(xyz+2).
Divide by x 3 on both sides
We get ,
y 3 + z 3 = x ² 2 0 1 2 y z + x ³ 4 0 2 4 ,
As clearly L.H.S is an integer ,
So , considering that if 2012yz/x² as an integer
Then 4024/x³ must be integer .
4024 = (8)(503).
So x must be 2.(minimum).
Then 2012yz/x² will be equal to 503yz which is also integer.
And L.H.S y 3 + z 3 ≥ 3yz. So it can be 503yz.
The assumption that both terms on the RHS (after division) must be integers need not be true. For example, 8 1 2 + 8 4 produces an integer.
The RHS is also an integer if x = 1.
Problem Loading...
Note Loading...
Set Loading...
First note that x divides 2012 · 2 = 2^3 · 503.
If 503 | x then the right-hand side of the equation is divisible by 503^3 , and it follows that 503^2 | xyz + 2. This is false as 503 | x. Hence x = 2^m with m ∈ {0, 1, 2, 3}.
If m ≥ 2 then 2^6 | 2012(xyz + 2). However the highest powers of 2 dividing 2012 and xyz + 2 = 2^myz + 2 are 22 and 21 respectively. So x = 1 or x = 2, yielding the two equations y^3 + y^3 = 2012(yz + 2), and y^3 + z^3 = 503(yz + 1).
In both cases the prime 503 = 3 · 167 + 2 divides y^3 + z^3.
We claim that 503 | y + z. This is clear if 503 | y, so let 503 ∤ y and 503 ∤ z. Then y 502 ≡ z 502 (mod 503) by Fermat’s little theorem. On the other hand y3 ≡ −z^3
(mod 503) implies y^3 167 ≡ −z^3 167 (mod 503), i. e.
y^501 ≡ −z^501 (mod 503). It follows that y ≡ −z (mod 503) as claimed. Therefore y + z = 503k with k ≥ 1. In view of y^3 + z^3
= (y + z)((y − z)^2 + yz) the two equations take the form k(y − z)^2 + (k − 4)yz = 8, (1) k(y − z)^2 + (k − 1)yz = 1. (2) In (1) we have (k − 4)yz ≤ 8, which implies k ≤ 4. Indeed if k > 4 then 1 ≤ (k − 4)yz ≤ 8, so that y ≤ 8 and z ≤ 8. This is impossible as y + z = 503k ≥ 503. Note next that y^3 + z^3
is even in the first equation. Hence y + z = 503k is even too, meaning that k is even. Thus k = 2 or k = 4. Clearly (1) has no integer solutions for k = 4. If k = 2 then (1) takes the form (y + z)^2 − 5yz = 4. Since y + z = 503k = 503 · 2, this leads to 5yz = 5032 · 2 2 − 4. However 503^2 · 2^2 − 4 is not a multiple of 5. Therefore (1) has no integer solutions. Equation (2) implies 0 ≤ (k − 1)yz ≤ 1, so that k = 1 or k = 2. Also 0 ≤ k(y − z)^2 ≤ 1, hence k = 2 only if y = z. However then y = z = 1, which is false in view of y + z ≥ 503. Therefore k = 1 and (2) takes the form (y − z)^2 = 1, yielding z − y = |y − z| = 1. Combined with k = 1 and y + z = 503k, this leads to y = 251, z = 252 In summary the triple (2, 251, 252) is the only solution. so x= 2