Math 1- arithmetic sequences

Examples of how to use the commands:

$t_1$
$t_{13}$
$\frac {n}{3}$
$2\cdot 3$

To give an example of a pattern here is an example:
7,13.5,20,26.5,33… find $t_{50}$ and $s_{50}$

$t_{50}$ = 1 + (50-1)6.5
$t_{50}$ = 1 + (49)6.5
$t_{50}$ = 319.5

$s_{50} = \frac{50}{2}$ (7 + 319.5)

$s_{50}$ = 8,162.5