①1999x^-(1999^-1)x-1999
=1999x^-1999^x+x-1999
=1999x(x-1999)+(x-1999)
=(1999x-1)(x-1999)
②(x^+xy+y^)^-4xy(x^y^)
=x^4+y^4+x^y^+2x^y^+2x^3y+2xy^3-4x^3y-4xy^3
=x^4+y^4+x^y^+2x^y^-2x^3y-2xy^3
=(x^-xy+y^)^
③x^3-3x+4
=（x^3-x^-6x）+(x^+4x+4)
=x(x-3)(x+2)+(x+2)^
=(x^-3x+x+2)(x+2)
=(x-2x+2)(x+2)
④4x^4+1
=4x^4++4x^+1-4x^
=(2x^+1)^-4x^
=(2x^+1-2x)(2x^+1+2x)
⑤x^+xy-6y^+x+13y-6
=(x+3y+3)(x-2y-2)
双十字相乘
⑥a^5+a^4+1
=a^5+a^4+a^3
-a^3-a^-a
+a^+a+1
=(a^+a+1)(a^3-a+1)