# I need to find the Sn for the arithmetic series with d = -4, an = 27, and n = 9

I need to find the Sn for the arithmetic series with d = -4, an = 27, and n = 9
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anekesanxl
We may use the formula for the sum to n terms of an arithmetic series :
$\sum _{k=1}^{n}\left[a+\left(k-1\right)d\right]=\frac{n}{2}\left[2a+\left(n-1\right)d\right]$
$=\frac{9}{2}\left[\left(2\right)\left(27\right)+\left(9-1\right)\left(-4\right)\right]$
$=99$