Question

HELP!! What is the twentieth term of the arithmetic sequence 21, 18, 15, 12, ... ?

1

78

-39

-36

Answer 1

Notice that each term is decreasing by 3 each time.
The common difference is -3.

To get to the 20th term, we'll subtract 3 a total of 19 times from the beginning value.

$\rm a_{20}=21+19(-3)\\ a_{20}=-36$