Question

calcule a soma dos primeiros : 10 termos da pa ( -20, -19, -18,... )

Answer 1

an = a1 + (n-1) . r
an = -20 + (10-1) . 1
an = -20 + 9 . 1
an = -29 + 9
an = - 20


Sn = (a1 + an) . n / 2
s10 = (-20 + -20) . 10 / 2
s10 = -40 . 10 / 2
s10 = -400 / 2
s10 = -200