Somebody help please!

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?

If you are just building your payment history, how many points from a perfect score will you possibly miss?

Acircle has a radius of 5/6 units and is centered at (3.6, 7.8) write the equation of this circle