a^4+b^4+c^4-2(ab)^2-2(ac)^2-2(bc)^2
=(a^4-2a²b²+b^4)+c^4-2a²c²-2b²c²
=(a²-b²)²+c^4-c²(2a²-2b²)
=(a+b)²(a-b)²+(c²)²-c²(a²+2ab+b²+a²-2ab+b²)
=(a+b)²(a-b)²+(c²)²-c²[(a+b)²+(a-b)²]
设(a+b)²=x,(a-b)²=y,c²=z
原式
=xy+z²-z(x+y)
=xy-zx+z²-zy
=x(y-z)-z(y-z)
=(x-z)(y-z)
=[(a+b)²-c²][(a-b)²-c²]
=(a+b-c)(a+b+c)(a-b+c)(a-b-c)