HI CAN SOMEONE HELP ME WITH THIS ILL GIVE U BRAINLIST

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?

The tile along the edge of a triangular community pool needs to be replaced? ?

If t8 = 4 and t12 = −2, find the first three terms of the arithmetic sequence.