is the same as and here you have your x.
2. You have to prove for each function if there exist any constraints of x such that the term f(x) can't be calculated.
The constraints you are supposed to know are:
i) The denominator of a fraction must unequal zero.
ii) The radical of a square-root must be non-negative.
iii) The argument of a logarithm-function must be positive.
3. There is only one function in your list whose domain isn't