Get the message can anyone tell me the message

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?

Select the lengths in centimeters that would form a right triangle

The ph level of a blueberry is 3.1 what is the hydrogen-ion concentration [h+] for the blueberry