never mind, i know how to find the equation now. what i still don't understand is what my book means by closure of a cone. is this the same closure as in the closure of a set X which is a subset of R^n? if X is described by y = x^3 then apparently the y axis is in the closure of the cone. i cannot see this geometrically as it is a slightly complicated surface. is there another way to explain the closure of a cone?