Choose the correct answer from the given four options:
The $ 21^{\text {st }} $ term of the AP whose first two terms are $ -3 $ and 4 is
(A) 17
(B) 137
(C) 143
(D) $ -143 $

Given:

First two terms of the A.P. are $–3$ and $4$.

To do:

We have to find the $21^{st}$ term of the A.P.

Solution:

$-3,\ 4,\ 11,\ 18\ .......$

Here, first term $a=-3$ 

Common difference, $D=7$

$n=21$

Now, $A_n=a+( n-1)d$

$\Rightarrow a_{21}=-3+20\times( 7)$

$\Rightarrow a_{21}=-3+140$

$\Rightarrow a_{21}=137$

Hence the $21^{st}$ term of the A.P. is $137$.