Question

Calcule a soma dos 6 primeiros termos da P.A (8,4, ...).

Answer 1

$a_6=a_1+5r\\\\ a_6=8+5*-4\\\\ a_6=8-20\\\\ \boxed{a_6=-12}$

Soma

$S_6=\frac{(a_1+a_6)*6}{2}\\\\ S_6=\frac{(8-12)*6}{2}\\\\ S_6=-4*3\\\\ \boxed{S_6=-12}$