pretty sure its 0
x = domain
y = range
the reason for that is the only numbers you can plug into the equation for it to work are between 0 and 1, say you want to plug in a 2...
f(2)= sqrt(1 - sqrt(1 - sqrt(2)))
you would get a negative sqrt because sqrt of 2 is 1.something and sqrt of (1 - 1.something) is negative, so x HAS to be between 1 and 0. you cant take a negative sqrt (unless you bring in imaginary numbers, and well im not getting into that).
now you plug in 0 for x and plug in 1 for x....
which gives you: f(1) = 1,
f(0) = 0
so y has to be between 0 and 1 because we plugged in both ends of the domain (x = 0, x = 1) and got those 2 answers.
basically you just want to find ALL of the possible answers that can make that equation work.
if you need any further clarification my aim is bk12321, wayyyy easier than posting on here