12x^3+x分之1-4x^4
=(1-2x²)(1+2x²)/[x(2x²+1)]
=(1-2x²)/x
2y-x^2y分之x^2y-2xy+y
=y(x-1)²/[y(1-x)(1+x)]
=(1-x)/(1+x)
3a^2-6ab+9b^2分之a^2-3ab
=a(a-3b)/[(a-3b)²]
=a/(a-3b)
4a^2-9b^2分之a+3b
=(a+3b)/[(a+3b)(a-3b)]
=1/(a-3b)
5a^3-4a^2b+4ab^2分之a^3-4ab^2
=a(a+2b)(a-2b)/[a(a-2b)²]
=(a+2b)/(a-2b)