-15ab的3次方/20a平方b =-5ab(b^2)/(5ab*4a) =-b^2/4a y+2/4-y平方 =(y+2)/(2-y)(2+y) =1/(2-y) 2x平方-18/x平方-6x+9 =2(x^2-9)/(x-3)^2 =2(x+3)(x-3)/(x-3)^2 =2(x+3)/(x-3) 2m-m平方/m平方-4m+4 =m(2-m)/(m-2)^2 =-m(m-2)/(m-2)^2 =-m/(m-2)