Question

Which term of an A.P $7,3,-1,...$ is $-89?$

Answer 1

Here $a=7,d=3-7=-4,t_n=-89$

$t_n=a+(n-1)d$

$\Rightarrow -89=7+(n-1)\times -4$

$\Rightarrow -89=7-4n+4$

$\Rightarrow -89=11-4n$

$\Rightarrow -4n=-89-11=-100$

$\Rightarrow n=\frac{-100}{-4}=25$

$\therefore {25}^{th}$ term of an A.P is $-89$