What is the pattern between these numbers 8,7,5,2

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?

Name each raycalculation tip: in ray "ab", a is the endpoint of the ray.

If you are trying to move an attached term across the equal sign (=), what operation would you use to move it?