Question

Calcule a soma dos vinte primeiros termos da P.A (-7,-3,1....)

Answer 1

a1=-7
a2=-3
razão=7-3=4
n=20

an=a1+(n-1).r
a20=-7+(20-1).4
a20=-7+19.4
a20=-7+76
a20=69


sn=(a1+a20).n/2
s20=(-7+69).20/2
s20=62.20/2
s20=1240/2
s20=620