Question

The first term of an A.P is $5$, the last term is $45$ and the sum is $400$. Find the number of terms in the A.P.

Answer 1

Given that, $a=5,l=45$ and $S_n=400$

$\Rightarrow$$S_n=\frac{n(a+l)}{2}$ .... Sum of nth terms

$\Rightarrow$$400=\frac{n(5+45)}{2}$

$\Rightarrow$$400=\frac{n\left(50\right)}{2}$

$\Rightarrow$$n=16$

the number of terms in the A.P. $n=16$