Question

Which term of the A.P 5,9,13,17.... is 81?

Answer 1

Given AP is $5,9,13,17,.....$

First term is $a=5$

Common difference $9-5=4$

$nth$ term is $a+(n-1)d5+(n-1)4=814(n-1)=81-54(n-1)=76n-1=19n=20$

So the $20th$ term is $81$