Question

In the series $3,9,15$, .., what will be the ${21}^{st}$ term ?

• A. $177$
• B. $121$
• C. $123$
• D. $129$

Answer 1

Answer: (C)

$123$

Clearly, $3+6=9,9+6=15,$

So, the series is an A.P. in which $a=3$ and $d=6.$

The terms can be written as $3,3+6,3+6\times 2,3+6\times 3,3+6\times 4,3+6\times 5,.....3+6\times 20$
$\therefore {21}^{st}$ term = $a+(21-1)d=a+20d$ = $3+20\times 6=123$.