The formula is .

For any input of , we get only one output for .

An input .

For any input of , we get only one output for .

So, the formula defines as function of .

 All possible values of is the domain of the function.

Above function is a radical function.

So the expression under the square root (the radicand) must be nonnegative.

(length of pendulum) is not equals to zero.

Solve the inequality

Domain set is .

The formula is a function.

Domain set is .