Question 290438
You can answer the question by finding the condition of a,b so that:
{{{sqrt(a^2 + b^2) = a + b}}} 
- The first condition is that (a+b) >= 0 since the left side of the equation is never negative.
- When both sides are both >= 0, we have:
{{{sqrt(a^2 + b^2) = a + b}}} <-> {{{a^2 + b^2 = (a+b)^2}}} <-> 2*a*b = 0 <-> a = 0 or b = 0.

Therefore, (1) is true with two conditions: (1) a + b >= 0 and (2) a*b = 0.
Is it clear now?