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Number theory Difficulty 5.8 AIME, harder Prove it Croatia

Determine all pairs (m,n)(m, n) of integers such that
n26n=m2+m10. n^2 - 6n = m^2 + m - 10.

Solution

We are given the equation
n26n=m2+m10. n^2 - 6n = m^2 + m - 10.
Let's rewrite it as
n26nm2m+10=0. n^2 - 6n - m^2 - m + 10 = 0.
Group terms:
n2m26nm+10=0. n^2 - m^2 - 6n - m + 10 = 0.
Recall that n2m2=(nm)(n+m)n^2 - m^2 = (n - m)(n + m), so
(nm)(n+m)6nm+10=0. (n - m)(n + m) - 6n - m + 10 = 0.
Let us solve for nn in terms of mm.

Alternatively, move all terms to one side:
n26nm2m+10=0 n^2 - 6n - m^2 - m + 10 = 0
n26n=m2+m10 n^2 - 6n = m^2 + m - 10
This is a quadratic in nn:
n26n(m2+m10)=0 n^2 - 6n - (m^2 + m - 10) = 0
The discriminant must be a perfect square for integer solutions:
Δ=36+4(m2+m10)=36+4m2+4m40=4m2+4m4=4(m2+m1) \Delta = 36 + 4(m^2 + m - 10) = 36 + 4m^2 + 4m - 40 = 4m^2 + 4m - 4 = 4(m^2 + m - 1)
So 4(m2+m1)4(m^2 + m - 1) must be a perfect square, say k2k^2 for some integer kk:
4(m2+m1)=k2 4(m^2 + m - 1) = k^2
So
k24m24m+4=0 k^2 - 4m^2 - 4m + 4 = 0
k24m24m+4=0 k^2 - 4m^2 - 4m + 4 = 0
k2=4m2+4m4 k^2 = 4m^2 + 4m - 4
k2=4(m2+m1) k^2 = 4(m^2 + m - 1)
So m2+m1m^2 + m - 1 must be a perfect square times 1/41/4.
But kk must be even, so let k=2tk = 2t for some integer tt:
(2t)2=4(m2+m1) (2t)^2 = 4(m^2 + m - 1)
4t2=4(m2+m1) 4t^2 = 4(m^2 + m - 1)
t2=m2+m1 t^2 = m^2 + m - 1
So m2+m1=t2m^2 + m - 1 = t^2 for some integer tt.

Now, rearrange:
m2+m1t2=0 m^2 + m - 1 - t^2 = 0
m2+mt2=1 m^2 + m - t^2 = 1
This is a quadratic in mm:
m2+m(t2+1)=0 m^2 + m - (t^2 + 1) = 0
The discriminant must be a perfect square:
Δ=1241(t21)=1+4t2+4=4t2+5 \Delta = 1^2 - 4 \cdot 1 \cdot (-t^2 - 1) = 1 + 4t^2 + 4 = 4t^2 + 5
So 4t2+54t^2 + 5 must be a perfect square, say s2s^2:
s2=4t2+5 s^2 = 4t^2 + 5
So s24t2=5s^2 - 4t^2 = 5

This is a Pell-type equation.

Let us solve s24t2=5s^2 - 4t^2 = 5 in integers.

Try small integer values for tt:
- t=0t = 0: s2=5s^2 = 5 (no integer solution)
- t=1t = 1: s2=4+5=9s^2 = 4 + 5 = 9, s=3s = 3
- t=1t = -1: s2=4+5=9s^2 = 4 + 5 = 9, s=3s = 3
- t=2t = 2: s2=16+5=21s^2 = 16 + 5 = 21 (no integer solution)
- t=2t = -2: s2=16+5=21s^2 = 16 + 5 = 21 (no integer solution)
- t=3t = 3: s2=36+5=41s^2 = 36 + 5 = 41 (no integer solution)
- t=3t = -3: s2=36+5=41s^2 = 36 + 5 = 41 (no integer solution)
- t=5t = 5: s2=100+5=105s^2 = 100 + 5 = 105 (no integer solution)

So only t=1t = 1 and t=1t = -1 work, with s=3s = 3.

Now, recall m2+mt2=1m^2 + m - t^2 = 1
For t=1t = 1:
m2+m1=1 m^2 + m - 1 = 1
m2+m=2 m^2 + m = 2
m2+m2=0 m^2 + m - 2 = 0
m=1±1+82=1±32 m = \frac{-1 \pm \sqrt{1 + 8}}{2} = \frac{-1 \pm 3}{2}
So m=1m = 1 or m=2m = -2

For t=1t = -1:
Same as above, m=1m = 1 or m=2m = -2

Now, recall nn is given by the quadratic:
n26n=m2+m10 n^2 - 6n = m^2 + m - 10
For m=1m = 1:
n26n=1+110=8 n^2 - 6n = 1 + 1 - 10 = -8
n26n+8=0 n^2 - 6n + 8 = 0
n=6±36322=6±22=4,2 n = \frac{6 \pm \sqrt{36 - 32}}{2} = \frac{6 \pm 2}{2} = 4, 2
For m=2m = -2:
n26n=4210=8 n^2 - 6n = 4 - 2 - 10 = -8
So same as above, n=4,2n = 4, 2

Therefore, the integer solutions are:
(m,n)=(1,2),(1,4),(2,2),(2,4)(m, n) = (1, 2), (1, 4), (-2, 2), (-2, 4)

Thus, all integer pairs (m,n)(m, n) such that n26n=m2+m10n^2 - 6n = m^2 + m - 10 are:
(1,2),(1,4),(2,2),(2,4)(1, 2), (1, 4), (-2, 2), (-2, 4)

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