Which term of the AP 21, 18, 15, .... is -81? - Vidyadip

Question

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

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Answer

The given AP is $21, 18, 15, ...$
$ a=21 \text { and } d=18-21=-3$
$a_n=a+(n-1) d$
$\Rightarrow-18=21+(n-1)(-3)$
$\Rightarrow-18=21+(n-1)(-3)$
$\Rightarrow-18=21-3 n+3$
$\Rightarrow 3 n=105$
$\Rightarrow n=35$
$\text { So, }-18 \text { is the } 35^{\text {th }} \text { term. }$

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