Question

The first and the last term of an $AP$ are $4$ and $81$ respectively. if the common difference is $7$, how many terms are there in the $AP$ and what is their sum?

Answer 1

Given that,

$a=4$

$d=7$

$a_n=81$

We know that,

$a_n=a+\left(n-1\right)d$

$81=4+\left(n-1\right)7$

$81=4+7n-7$

$7n=81+3$

$n=\frac{84}{7}=12$

Then,

We get $A.P$of sum $12$