# Math Help - find the sum of the arithmetic equation.

1. ## find the sum of the arithmetic equation.

an = 5
d = -2
n = 12

2. For the first n terms the formula is given by:

$S_n = \frac{n}{2}(2a + (n-1)d)$

or

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

where l is the last term that you are counting in.

Post what you get.