Which function can be used to find the nth term of the arithmetic sequence?

So, the first term in the arithmetic sequence is -1, then the fourth is -4, the tenth is -10... h

notice, an arithmetic sequence has a "additioner", or a "common difference", from -1 to 3 terms later it ends up as -4, so the common difference is -1.

so, therefore the sequence is then -1, (-1-1) = -2, (-2-1)=-3, (-3-1)= -4 and so on.

so f(-1) = -1, f(4) = -4, f(10)  = -10, thus f(n) = -n