Question

First 2 terms of an ap -1 and 1, then 21st term is??​

Answer 1

GIVEN :-

$\sf{\ a_{1}=-1}$

$\sf{\ a_{2}=1}$

$\sf{d=1-(-1) = 2 }$

SOLUTION :-

• Firstly , we find the value of common difference with the help of first and second term. Now we will directly use the formula to find the 21st term

• FORMULA USED

$\sf{\ a_{n}=a+(n-1)d}$

$\sf{\ a_{21}=a+(21-1)d}$

$\sf{\ a_{21}=a+20d}$

$\sf{\ a_{21}=-1+20\times2}$

$\sf{\ a_{21}=40-1}$

$\sf{\ a_{21}=39}$

So , the 21st term of the ap is 39