5/(-2a),2/(9a^2b^3)
－45ab^3/(18a^2b^3),4/(18a^2b^3),约分（3x的立方+6x的平方y）分之（x的立方-4xy的平方）（x^3-4xy^2)/(3x^3+6x^2y）=x(x^2-4y^2)/[3x^2(x+2y)]=(x+2y)(x-2y)/[3x(x+2y)]=(x-2y)/(3x)