Write a formula for the general term (the nth term) of the arithmetic sequence shown below. Do not use a recursion formula. Then use the formula for an to find a20, the 20th term of the sequence.

a1 = - 13, d = - 5

an = a1 + (n-1) * d

a20 = - 108

Step-by-step explanation:

For the arithmetic sequence

a1 = - 13, d = - 5

a2 = a1 + d (where d is the ratio of the sequence)

a2 = - 13 - 5

a2 = - 18

a3 = a2 + d

But a2 = a1 + r

a3 = a1 + 2r = - 13 + 2 (-5) = - 23

So: an = a1 + (n-1) * d

a20 = - 13 + 19 * (-5)

a20 = - 108