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I have the following expression

 FullSimplify[
 Sqrt[1 + (-1 + a) (-1 + b) + 2 Sqrt[(-1 + a) (-1 + b)]] - Sqrt[
  2 + a (-1 + b) - 2 Sqrt[(-1 + a) (-1 + b)] - b]]

Where $a\ge1$ and $b\ge1$. I know the answer should be $\sqrt{(1-a)(1-b)}$. However, $Mathematica$ doesn't solve this expression this way. How can one resolve this?

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2
  • $\begingroup$ Reduce[{Sqrt[1 + (-1 + a) (-1 + b) + 2 Sqrt[(-1 + a) (-1 + b)]] - Sqrt[2 + a (-1 + b) - 2 Sqrt[(-1 + a) (-1 + b)] - b] == Sqrt[(a - 1) (b - 1)], a > 1, b > 1}] says this is True only when a == (3 + b)/(-1 + b) $\endgroup$
    – kglr
    Feb 19, 2019 at 9:28
  • $\begingroup$ They don't seem to match at, for example, a -> 1.23, b -> 2.34. $\endgroup$ Feb 19, 2019 at 9:28

1 Answer 1

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Try this:

 expr1 = Sqrt[1 + (-1 + a) (-1 + b) + 2 Sqrt[(-1 + a) (-1 + b)]] - 
      Sqrt[2 + a (-1 + b) - 2 Sqrt[(-1 + a) (-1 + b)] - b];
expr2 = Simplify[expr1 /. {a -> x + 1, b -> y + 1}, {x > 0, y > 0}]

(*  -Sqrt[1 + x y - 2 Sqrt[x y]] + Sqrt[1 + x y + 2 Sqrt[x y]]  *)

Then

expr3=Simplify[expr2 /. x*y -> z^2, z > 0]

enter image description here

Now

expr3 /. z -> (a - 1) (b - 1)

yields

enter image description here

Have fun!

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