Notice that which means x1x2-2x2+2x1-4 = x1x2+2x2-2x1-4. It follows that x1=x2 and so f is injective. To find the inverse, put y=(x-2)/(x+2). This can be written as xy+2y=x-2 or x=2(1+y)/(1-y). Thus f-1(x)=2(1+x)/(1-x).
