Choose the correct answer from the given four options : The 21^ s… - Vidyadip

MCQ

Choose the correct answer from the given four options : The $21^{st}$ term of the $AP$ whose first two terms are $–3$ and 4 is :

A
$17$
✓
$137$
C
$143$
D
$-143$

✓

Answer

Correct option: B.

$137$

Main concept used: $a_n= a + (n - 1)d$
Here, $a=a_1=-3, a_2=4$
$\therefore d=a_2-a_1=4-(-3)=4+3=7$
Hence, $d=7$
Now, $a_n=a+(n-1) d$
$ \Rightarrow a_{21}=-3+(21-1) \times 7 $
$ \Rightarrow a_{21}=-3+20 \times 7 $
$ \Rightarrow a_{21}=-3+140 $
$ \Rightarrow a_{21}=137 $

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