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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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