How do you find the domain of #f(x) = 10-x^2#?
The domain of the function is the largest subset of real numbers
To find the domain you have to look at the function's formula to find numbers, for which it cannot be calculated. Such points could be:
zeros of denominators,
points where values under root sign are negative,
points where expressions under
#log#signs are negative
and so on.
The example given does not contain any of the above, so its domain is the whole set of real numbers.