x y = y 3 + 1 0 x + x 2
If the equation above have positive integer solution of x = α , y = β , then find the value of α β .
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@Danny Kills @Calvin Lin Do you have a more elegant way of finding natural solutions to the last equation i.e., 2 y = y 3 + 2 4 ? If so, do you mind sharing it? Thanks!
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To find solutions to 2 y = y 3 + 2 4 , I would observe that 2 y grows exponentially while y 3 is polyonmial, hence we have 2 y > y 3 + 2 4 for y > 1 1 . We then need to just test a few small cases.
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Do you say 2 y > y 3 + 2 4 for y > 1 1 by observation or there is any method ? Thanks.
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@Niranjan Khanderia – By observation. Since exponential growth is much faster than polynomial growth, I know that such a "boundary" value of Y exists. It just remains to use trial and error to get there. In such a case, it doesn't really matter if you have to try 10 values, or 20 values.
If we have access to a graph of x l n x , then we can compare that to 3 ln 2 to easily get what the limiting value would be.
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@Calvin Lin – Thanks. Good to know that trial an error is also OK. I often use it and then try to reason out from the collected results..
Note that the first conclusion is not true. Even though x divides the RHS, that just means that x ∣ y 3 . It does not mean that x ∣ y . For example, take x = 8 , y = 2 .
Of course, x ∣ y is implicit in the question, given that x y is an integer. But that is besides the point.
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@Calvin Lin Thanks for pointing that out. I'll try to think of another solution. BTW can you tell me how did you find in this problem that the circle with center ( 1 3 , 1 3 ) and radius 1 3 when tangent to the line will give the minimum value? I thought about ex-circles but couldn't expand upon my idea. Can you please help? Thanks again!
We are given x y = y 3 + 1 0 x + x 2 .
If
y
=
1
, then clearly
L
H
S
<
R
H
S
.
If
y
=
2
, then clearly
L
H
S
<
R
H
S
.
Henceforth,
y
≥
3
.
If x ≥ 4 , then for y ≥ 3 , we have x y > y 3 + 1 0 x + x 2 , so no solutions.
If x = 3 , then for y ≥ 4 , we have 3 y > y 3 + 3 9 . For y = 3 , we have 2 7 < 2 7 + 3 9 , so no solutions.
If x = 2 , then we have 2 y = y 3 + 2 4 . For y ≥ 1 1 , we have 2 y > y 3 + 2 4 . Hence, we only need to check y = 1 to 10, of which y = 1 0 works.
If x = 1 , then we have 1 = y 3 + 1 0 + 1 which has no solutions.
Hence, ( 2 , 1 0 ) is the only solution.
If the equation would have been x y = y 3 + 2 y + x 2 ?
Clearly, x is the smaller number. If x=1,
y
3
=10, not an integer. x=2, we have,
f
(
x
,
y
)
=
y
3
+
1
0
x
+
x
2
−
x
y
=
0
,
∴
f
(
2
,
y
)
=
y
3
+
2
0
+
4
−
2
y
=
0
,
⟹
y
3
=
2
y
−
2
4
.
y
>
1
,
∴
2
y
>
2
4
,
⟹
y
>
4
.
L
e
t
g
(
y
)
=
2
y
−
2
4
.
Since y is a +tive integer,
and answer box excepts only integers, we check for y=6,8,10.
g
(
1
0
)
=
1
0
2
4
−
2
4
=
1
0
0
0
=
1
0
3
.
r
e
q
u
r
e
d
r
a
t
i
o
=
2
1
0
=
5
‘
‘
‘
We can try to get a rough estimate as follows.
X
Y
>
Y
3
∴
Y
l
o
g
X
>
3
l
o
g
Y
,
⟹
l
o
g
Y
Y
>
l
o
g
X
3
.
Draw two graphs, one for the left and other for the right.
They intersect near integer x=3 ONLY.
Since it is inequality we go for x=2. Then go as above.
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Given: x y = y 3 + 1 0 x + x 2
⟹ x y − 1 0 x − x 2 = y 3
⟹ x ∣ y
Let y = k x
Putting this in our equation, we get,
x k x − 1 0 x − x 2 = k 3 x 3
⟹ x k x − 3 − x 3 ( 1 0 x + x 2 ) = k 3 or y = 1 , 2 or x = 0
Checking y = 1 , 2 we don't get natural x , so we reject y = 1 , 2
Also, for x = 0 , we get y = 0 , but putting both of them together in the original equation will yield 0 0 in L.H.S. , which is undefined, so we reject x = 0
⟹ x k x − 3 − x 3 ( 1 0 x + x 2 ) = k 3
⟹ x k x − 3 − x 2 ( 1 0 + x ) = k 3 (Cancelling x from Numerator and Denominator)
⟹ x 2 ∣ ( 1 0 + x )
⟹ x 2 ≤ ( x + 1 0 )
For natural x , we get x = 1 or x = 2 as the only numbers satisfying the restrictions.
If x = 1 ,
⟹ y 3 = − 1 0
Thus rejecting x = 1 , we get x = 2 and
2 y = y 3 + 2 4
We can easily plot the graph of L.H.S. and R.H.S. on the same cartesian plane and see that there are exactly two points of intersection out of which one gives negative value of y and the other one is y = 1 0 (To do this easily some calculus can be used).
Thus β = 1 0 , α = 2
⟹ α β = 5 .
Alternate solution
Like above, we find x = 2 and get
2 y = y 3 + 2 4
Now,
Since L.H.S. of the equation is even ⟹ R.H.S. is even
⟹ y is even.
Let y = 2 k
⟹ 2 2 k = 8 k 3 + 2 4
⟹ 2 2 k − 3 = k 3 + 3 (Since y ≥ 3 from first part i.e. of finding the value of x )
Since L.H.S. of the equation is even ⟹ R.H.S. is even
⟹ k is odd.
Again rewriting the equation, we get,
4 k = 8 k 3 + 2 4
Also, since
4 ≡ ( − 1 ) m o d 5
⟹ 4 k ≡ ( − 1 ) k ≡ ( − 1 ) m o d 5 (Since k is odd)
⟹ 8 k 3 + 2 4 ≡ ( − 1 ) m o d 5
⟹ k ≡ 0 m o d 5
Let k = 5 j
⟹ ( 4 5 ) j = 1 0 0 0 j 3 + 2 4
⟹ ( 1 0 2 4 ) j = 1 0 0 0 j 3 + 2 4
Now, it is trivial to see that j = 1 is a solution giving y = 2 and for higher values of j we can see even without graph that L.H.S. increases much more rapidly than R.H.S. . Further, we do not need to analyse for j < 1 since that would give non-natural y .
Therefore, again we get
β = 1 0 , α = 2
⟹ α β = 5 .