In the figure, \triangle ABC is a right triangle in which, \angle ACB = 90\degree, AB=13, CD\parallel AB, point E is a dynamic point in the ray of CD ( not coincide with point C ) , connect AE, intersect BC at point F, \angle BAE bisects BC at point G。

If CE=3, find the ratio of areas of triangles S_{\triangle CEF} : S_{\triangle CAF };

Let CE=x, AE=y. If CG=2GB, find function y in terms of x;