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HELP!! What is the twentieth term of the arithmetic sequence 21, 18, 15, 12, ... ? 1 78 -39 -36
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4 votes
HELP!! What is the twentieth term of the arithmetic sequence 21, 18, 15, 12, ... ?

1

78

-39

-36

User Adam Crume
by
6.5k points

1 Answer

4 votes
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
User Dcromley
by
7.4k points
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