We know the following relationship:
The domain of a function are the inputs of the function, that is, a function
is a relation that assigns to each element 
in the set A exactly one element in the set B. The set A is the domain (or set of inputs) of the function and the set B contains the range (or set of outputs).Then applying this concept to our function 
we can write its domain as follows:
1. D<span>omain of validity for:
</span>
When:
?
when:
where k is an integer either positive or negative. That is:
To match this with the choices above, the answer is:
<span>"All real numbers except multiples of
"
</span>
2. which identity is not used in the proof of the identity
:
This identity can proved as follows:
The identity that is not used is as established in the statement above:
<span>"1 +cos squared theta over sin squared theta= csc2theta"
Written in mathematical language as follows:
</span>
<span>
</span>
The domain of a function are the inputs of the function, that is, a function
1. D<span>omain of validity for
</span>
When:
when:
where k is an integer either positive or negative. That is:
To match this with the choices above, the answer is:
<span>"All real numbers except multiples of
</span>
2. which identity is not used in the proof of the identity
This identity can proved as follows:
The identity that is not used is as established in the statement above:
<span>"1 +cos squared theta over sin squared theta= csc2theta"
Written in mathematical language as follows:
</span>
</span>