Find the $21^{st}$ term of the A.P. whose first two terms are $–3$ and $4$.


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

To do: 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. will be $137$.

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Updated on: 10-Oct-2022

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