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Find the middle term(s) of the A.P. $9, 15, 21, \dots, 243$.
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$a_n = 9 + (n-1) 6 = 243$
$\implies n = 40$
$a_{20} = 9 + 19 \times 6 = 123$
$a_{21} = 9 + 20 \times 6 = 129$
$\therefore$ The two middle terms are $123$ and $129$
$\implies n = 40$
$a_{20} = 9 + 19 \times 6 = 123$
$a_{21} = 9 + 20 \times 6 = 129$
$\therefore$ The two middle terms are $123$ and $129$