What is the missing constant term in the perfect square that starts with x2 + 18x

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?

What is the distance between point a and b

What is the measure of one interior angle of a pentagon