Question 937688
((x)/(1-x)) - ((y)/(1-y)) 
use (1-x)*(1-y) as the common denominator, then
x(1-y) - y(1-x) / (1-x)(1-y)
(x-xy - y +xy) / (1-x)(1-y)
(x-y) / (1-x)(1-y)
((x-y)/(xy-x-y+1))