Question

Make 'd' the subject of formula :
$s=\frac{n}{2}[2a+(n-1)d]$.
Hence, find 'd', if s = 144, a = 1 and n = 12.

Answer 1

Given, $s=\frac{n}{2}[2a+(n-1\left)d\right]$
$=>\frac{2s}{n}=2a+(n-1)d$
$=>\frac{2s}{n}-2a=(n-1)d$
$=>\frac{2s-2an}{n}=(n-1)d$
$=>d=\frac{2s-2an}{n(n-1)}$
When $s=144,a=1,n=12$
$d=\frac{2\times 144-2\times 1\times 12}{12(12-1)}$
$d=\frac{288-24}{12\times 11}=2$