Simplify $\frac{a+b-\sqrt{a^2-b^2}}{\sqrt{a+b}}$ I have to simplify the following expression:
$A =\frac{a+b-\sqrt{a^2-b^2}}{\sqrt{a+b}}$

Answer: $\sqrt{a+b}-\sqrt{a-b}$

I am trying to find the constraints of $a$ and $b$. I think that $a^2-b^2 \ge 0$ and $a+b \ge 0$. How can I simplify them? (the inequalities)

1)  You have $\sqrt{a + b}$ in the denominator before simplification, so you can not have $\sqrt{a+ b} = 0$.  So $a+b \ne 0$ and $a \ne -b$.
2) You have $\sqrt{a+b}$ before simplification so $a+b \ge 0$ and by 1) you have $a+b \ne 0$ so $a+ b > 0$ or $a > -b$.  
3) You have $\sqrt{a^2 - b^2}$ before simplification so $a^2 -b^2 \ge 0$.
$a^2 - b^2 = (a+b)(a-b)$ and by 2) we have $a+b > 0$ so $a-b \ge 0$ so $a \ge b$.
We have $a \ge b$ and $a > -b$ so $a \ge |b|$.  
If $b < 0$ then $-b > 0$ and $a > -b > 0$.  If $b=0$ then $a > -b = 0$.  If $b> 0$ then $a \ge |b| =b > 0$ so we have:

$a > 0$ and $a \ge |b|$ and if $b < 0$ then $a > |b|$.

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Alternatively  $a^2 -b^2 \ge 0$ so $a^2 \ge b^2$ so $|a| \ge |b|$.
but $a + b > 0$ so $a > -b$ and .... same results of above.
