x的4次方-x的平方
=x²(x²-1)
=x²(x+1)(x-1)
a(x的平方-y的平方)+b(y的平方-x的平方)
=a(x²-y²)-b(x²-y²)
=(x²-y²)(a-b)
=(x+y)(x-y)(a-b)
-27x的4次方+18x的3次方-3x的平方
=-3x²(9x²-6x+1)
=-3x²(3x-1)²
x的4次方-1
=(x²+1)(x²-1)
=(x²+1)(x+1)(x-1)