I'll explain the first question. You have (is that right ? There is confusion in your question, is it the whole fraction under square root ?) :
Now you want to find the domain of this function. That is, for which values of is defined.
Step 1 - look out for any special operations that are not defined for all real numbers (for example, is defined for any real number, but is not defined for negative numbers).
Here, you have two such operations : the square root cannot take any negative number, and the denominator of the fraction cannot be equal to .
Step 2 - identify the expressions that may lead to an undefined function.
First issue, the square root : cannot be negative, so it must be positive or equal to . Thus, we have :
Second issue, the denominator, which is , cannot be zero. Thus, we have :
Step 3 - put together the information and extract the domain.
Remind the information found :
Therefore, the domain of would be :
Step 4 - check your answer on a graph-drawer software (or your calculator). Ensure the domain you found matches the one found by the software, and be sure you put the right interval signs ( [,],(,) ), especially on a zero-denominator restriction (be sure to exclude it on BOTH sides of the interval).
Does it help ?
PS : if I misunderstood your equation, I made sure I gave you enough tips for you to be able to work out the domain by yourself following the 4 steps.