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Which term of the AP $21, 18, 15, \ldots$ is $-81$?

A33rd
B34th
C35th
D36th
Answer & Solution
Correct answer: C. 35th
For the AP, $a=21$ and $d=-3$. Let $a_n=-81$. Then $$-81=21+(n-1)(-3).$$ So $-81=24-3n$, hence $-105=-3n$ and $n=35$. Therefore, $-81$ is the 35th term.
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