What is the perimeter of the polygon found

1. what are the missing reasons in the proof 2. it's in the second picture and 3

One expression below in undefined, and the other expression has a well defined value. which expression is undefined and explain why it has no value. which expression is defined? what is the value of that expression and how do you know? what is a different expression using a trig inverse function that is also undefined?

Solve for x and select the correct answer