Question

calcule a soma dos vinte primeiro termo da p.a (3,7,11,15,...

Answer 1

An=a1+(n-1).r
A20=3+(20-1).4
A20=3+19.4
A20=3+76
A20=79
Sn=(a1+an).n/2
S20=(3+79).20/2
S20=82.20/2
S20=1640/2
S20=820