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Algebra Difficulty 5.4 AIME, harder Prove it Hong Kong

Find, with reasons, all integers aa, bb, and cc such that
12(a+b)(b+c)(c+a)+(a+b+c)3=1abc. \frac{1}{2}(a+b)(b+c)(c+a) + (a+b+c)^3 = 1 - abc.

Solution

The solutions are (a,b,c)=(1,0,0),(2,1,1)(a, b, c) = (1, 0, 0), (2, -1, -1) up to permutation.
Let s=a+b+cs = a + b + c. Consider
P(x)=(xa)(xb)(xc)=x3sx2+(ab+bc+ca)xabc. P(x) = (x - a)(x - b)(x - c) = x^3 - sx^2 + (ab + bc + ca)x - abc.
Putting x=sx = s, we obtain (b+c)(c+a)(a+b)=(ab+bc+ca)sabc(b+c)(c+a)(a+b) = (ab+bc+ca)s - abc. Therefore, the given equation becomes
(ab+bc+ca)sabc+2s3=22abc. (ab + bc + ca)s - abc + 2s^3 = 2 - 2abc.
This is the same as s3s3(ab+bc+ca)sabc=2-s^3 - s^3 - (ab + bc + ca)s - abc = -2, i.e. P(s)=2P(-s) = -2. In other words, we have
(2a+b+c)(a+2b+c)(a+b+2c)=2. (2a + b + c)(a + 2b + c)(a + b + 2c) = 2.
(One can prove this by direct expansion.) By symmetry, it suffices to consider a few cases as follows.
* If 2a+b+c=22a + b + c = 2 and a+2b+c=a+b+2c=1a + 2b + c = a + b + 2c = 1, adding these we obtain 4(a+b+c)=44(a + b + c) = 4. So we have a+b+c=1a + b + c = 1, and hence (a,b,c)=(1,0,0)(a, b, c) = (1, 0, 0).
* If 2a+b+c=22a + b + c = 2 and a+2b+c=a+b+2c=1a + 2b + c = a + b + 2c = -1, adding these we obtain 4(a+b+c)=04(a + b + c) = 0. So we have a+b+c=0a + b + c = 0, and hence (a,b,c)=(2,1,1)(a, b, c) = (2, -1, -1).
* If 2a+b+c=22a + b + c = -2, a+2b+c=±1a + 2b + c = \pm 1 and a+b+2c=1a + b + 2c = \mp 1, adding these we obtain 4(a+b+c)=24(a + b + c) = -2. There is no integer solution.
This shows the only solutions are (a,b,c)=(1,0,0),(2,1,1)(a, b, c) = (1, 0, 0), (2, -1, -1) up to permutation.

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