1) (√x+√y)/(√x-√y)=

= (√x+√y)²/[(√x-√y)·( √x+√y)] =

= (√x+√y)²/[(√x)²-(√y)²] =

= (√x+√y)²/(x-y)

2) (20-4√a)/(5√a-a) =

= 4(5 - √a)/(5√а - √a·√a)

= 4(5 - √a)/[√a(5 -√a)] =

= 4/√a =

= 4√a/(√a·√a) =

= 4√a/a

3) (9√a+√b)/(9b+81√ab) =

= (9√a+√b)/[9√b(√b+9√a]) =

= 1/(9√b) =

= √b/(9b)

4) (x-a√x)/(√ax-a√a) =

= √x(√x - a)/[√a(√x - a)] =

= √x/√a =

= √(ax)/a