Question

6) Determine a soma dos 80 primeiros termos da P.A. (-10, -14, -18, ...). *
6 pontos
(A) - 28.080
(B) -26.080
(C) -326
(D) 79
(E) 80

Answer 1

☑️ Encontrar a razão da PA

• $\small{r=a2-a1}$

• $\small{r=-14-(-10)}$

• $\small{r=-14+10}$

• $\small{r=-4}$

☑️ Encontrar o octagesimo

• $\small{a80=a1+79r}$

• $\small{a80=-10+79.(-4)}$

• $\small{a80=-10+316}$

• $\small{a80=306}$

☑️ Encontrar a soma ( s80)

• $\small{sn=\dfrac{n.(a1+an)}{2}}$

• $\small{s80=\dfrac{80.(-10+306)}{2}}$

• $\small{s80=40.(296)}$

• $\underbrace{\small{s80=11840}}$

☑️ Essa soma será igual 11840