-a^4-b^4-c^4+2a^2b^2+2b^2c^2+2c^2a^2=-[a^4+2a²b²+b^4-2a²c²-2b²c²+c^4-4a²b²]=-[(a²+b²)²-2c²(a²+b²)+c^4-(2ab)²]=-[(a²+b
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