a,a+d,a+2d,...

a,ad,a2d,...

a,ar,a $r^2$ ,...

a,a+r,a+ $r^2$ ,..

$tn_{ }=a-\left(n+1\right)d$

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

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

$n=\frac{\left(l+a\right)}{d}+1$

$n=\frac{\left(l-a\right)}{d}+1$

$n=\frac{\left(l+a\right)}{d}-1$

$n=\frac{\left(l-a\right)}{d}-1$