3)(a^2 - ax)/(a^2 x - ax^2) - 1/x = a(a - x)/ax(a - x) - 1/x = 1/x - 1/x = 0
4)(mn^4 - an^4)/(an^3 - mn^3) + 2n = -n^4(a - m)/n^3(a - m) + 2n = -n + 2n = n
5) (x^2 - 1)/(1 - x^3) - x^2/(x^2 + x + 1) = -(x - 1)(x + 1)/(x - 1)(x^2 + x + 1) - x^2/(x^2 + x + 1) = ( -x - 1 - x^2)/(x^2 + x + 1) = -(x^2 + x + 1)/(x^2 + x + 1) = -1
6) (x^4 - y^4)/(x^3 - yx^2 + xy^2 - y^3) - 2y = (x - y)(x + y)(x^2 + y^2)/((x - y)(x^2 + xy + 1) - xy(x - y)) - 2y = (x - y) (x + y)(x^2 + y^2)/((x - y)(x^2 + xy - xy + y^2) - 2y = (x - y) (x + y)(x^2 + y^2)/((x - y)(x^2 + y^2) - 2y = x + y - 2y =x - y