Data Sufficiency

Question the first:

Are x and y both positive?

1) 2x - 2y = 1
2) x/y > 1

The answer is that both statements together are sufficient but neither statement alone is sufficient...can someone please show me how to arrive at this conclusion?

Question the second:

If a and b are positive numbers, what are the coordinates of the midpoint of line segment CD in the xy-plane?

1) The coordinates of C are (a, 1-b)
2) The coordinates of D are (1-a, b)

The answer is that both statements together are sufficient but neither statement alone is sufficient...is it enough to express the midpoint using a and b as follows?

((1-a)-a)/2 , (b-(1-b))/2 --->
(1-2a)/2 , (2b-1)/2