Question

Calcule os 20 primeiros termos da PA (-2 , -4 , -6...)

Answer 1

Bom Dia!

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Dados;

A1 → -2

N → 20

R → a2-a1 → -4-(-2) → -4+2 = -2

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An=a1+(n-1)·r

A20=-2+(20-1)·(-2)

A20=-2+19·(-2)

A20=-2-38

A20=-40

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$\textit{Sn}=\frac{\mathrm{(A1+An).n} }{\mathrm{2}}$

$\textit{S20}=\frac{\mathrm{(-2+(-40).15}}{\mathrm{2}}$

$\textit{S20}=\frac{\mathrm{(-2-40).15}}{\mathrm{2}}$

$\textit{S20}=\frac{\mathrm{-42.15}}{\mathrm{2}}$

$\textit{S20}=\frac{\mathrm{-630}}{\mathrm{2}}$

$\mathbf{S20}=\boxed{-315}$

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Att;Guilherme Lima