Question
Which term of the AP 21, 18, 15, .... is -81?
✓
Answer
The given AP is 21, 18, 15, ...
a = 21 and $d = 18 - 21 = - 3$
$a_n = a + (n - 1)d$
$\Rightarrow -18 = 21 + (n - 1)(-3)$
$\Rightarrow -18 = 21 + (n -1)(-3)$
$\Rightarrow -18 = 21 - 3n + 3$
$\Rightarrow 3n = 105$
$\Rightarrow n = 35$
So, -18 is the $35^{th}$ term.
a = 21 and $d = 18 - 21 = - 3$
$a_n = a + (n - 1)d$
$\Rightarrow -18 = 21 + (n - 1)(-3)$
$\Rightarrow -18 = 21 + (n -1)(-3)$
$\Rightarrow -18 = 21 - 3n + 3$
$\Rightarrow 3n = 105$
$\Rightarrow n = 35$
So, -18 is the $35^{th}$ term.
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