Question

In an AP l=28, Sn=144, n=9. Find a?

Answer 1

use the formula of :
sum = n/2(a+l)
144=9/2(a+28)

Answer 2

$S_n = \frac{n}{2} \bigl ( a+l \bigr)$

Given that:

S = 144, l = 28 , n = 9 

⇒ $144 = \frac{9}{2} (a+28)$

⇒ $\frac{144\times 2}{9} =a+28$

⇒ $a = 4.$