Whats the opposite of a number called?

How do you find the area of this triangle.

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?

1. the wheeling bridge in west virginia is about 307 meters long. if you walk with a stride of about meter, about how many steps would it take you to cross this suspension bridge?