Question

What is the ${21}^{st}$ term of the arithmetic sequence $-5,-2,1,4,7......?$

• A. $51$
• B. $55$
• C. $54$
• D. $53$

Answer 1

Answer: (B)

$55$

Given sequence is $-5,-2,1,4,7.........$
To find out: The ${21}^{st}$ term of the sequence.

We know that, the $n^{th}$ term of an A.P. is $t_n=a+(n-1)d$

Here $a=-5,n=21andd=3$

$\therefore t_{21}=-5+(21-1)3$
$\Rightarrow -5+\left(20\right)3$
$\Rightarrow -5+60$
$\therefore t_{21}=55$

Hence, the ${21}^{st}$ term of the given sequence is $55$.