$r=\frac{t_2}{t_1}$  

 $d=t_2-t_1$  

 $d=t_1+t_2$  

$t_n=ar^{n-1}$

$t_n=a+r^n-1$

$t_n=ar^n-1$

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

$b^2=ac$

$b^2=a+c$

$a^2=b+c$

$2b=ac$

$s_n=a\left(r^n-1\right)$

$S_n=\frac{a\left(r^n-1\right)}{r-1}$

$S_n=\frac{n}{2}\lceil a+l\rceil$

$S_n=ar^{n-1}$

$\frac{a}{1-r}$

$a\left(1-r\right)$

$ar^{n-1}$

$\frac{a}{1-n}$

n(n+1)

n(n-1)

$\frac{n\left(n+1\right)}{2}$

$\frac{n\left(n-1\right)}{2}$

$\frac{n\left(n+1\right)}{2}$

$\frac{n\left(n+1\right)\left(2n+1\right)}{2}$

$\frac{n\left(n+1\right)\left(2n+1\right)}{6}$

$\frac{n\left(n+1\right)\left(n+2\right)}{6}$

$\frac{n\left(n+1\right)}{2}$

$\left[\frac{n\left(n+1\right)}{2}\right]^2$

$n^2$

$\left[\frac{n\left(n+1\right)}{2}\right]^3$

$n^2$

$2n$

$2n-1$

$\frac{n\left(n+1\right)}{2}$