The ellipse E has equation x^2/a^2+y^2/b^2=1 and the line L has equation =mx+c, where m>0 and c>0. If L is a tangent to the ellipse and meets the negative x-axis at the point A and the positive y-axis at the point B, and O is the origin,
(i) Prove that, as m varies, the minimum area of triangle OAB is ab.
(ii) Find, in terms of a, the x-coordinate of the point of contact of L and E when the area of triangle OAB is a minimum.