Originally Posted by
Krizalid Well, it's not that hard to find the "why."
Let's use our brain: having $\displaystyle f(x)=\sqrt x.$ For what values of $\displaystyle x,$ $\displaystyle \sqrt x$ is well defined? Yes, we require that $\displaystyle x\ge0,$ 'cause $\displaystyle x$ can be zero, or positive, but it can't be negative, since square roots of negative numbers don't belong to the Real numbers, that's why we require $\displaystyle x\ge0.$
Now, by applyin' this to your problems, you'll see this does make sense.