Question

Calcular a soma dos 30 primeiros termos da P.A (-2;-4;-6...)

Answer 1

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$a_n=a_1+(n-1)r \\ \\ a_n=-2+(30-1)(-2) \\ \\ a_n=-2+(29)(-2) \\ \\ a_n=-2-58 \\ \\ a_n=-60$


$S_n= \frac{n(a_1+a_n)}{2}$

$S_{30}= \frac{30(-2+(-60))}{2} \\ \\ S_{30}=15(-2-60) \\ \\ S_{30}=15(-62) \\ \\ S_{30}=-930$