x(x+1)(x+2)(x+3)-8
=x(x+3)(x+1)(x+2)-8
=(x^2+3x)(x^2+3x+2)-8
=x^2+3x)[（x^2+3x)+2]-8
=(x^2+3x)^2+2(x^2+3x)-8
=(x^2+3x+4)(x^2+3x-2)
x^2-xy-2y^2-x+8y-6
=(x-2y)(x+y)-3(x-2y)+2(x+y)-6
=[(x-2y)+2][(x+y)-3]
=(x-2y+2)(x+y-3)
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