2-18a^4 分之 6a^2b^2-2b^2
=(6a^2b^2-2b^2)/(2-18a^4)
=2b^2(3a^2-1)/2(1-9a^4)
=b^2(3a^2-1)/(1+3a^2)(1-3a^2)
=-b^2/(1+3a^2)