Question

Calcule a soma dos 20 primeiros da p.a ( 4,8,...)

Answer 1

A1=4
R=8-4=4

A20=A1+19R

A20=4+19(4)

A20=4+76

A20=80

Sn=(A1+A20)N/2

Sn=(4+80)20/2

Sn=84.10

Sn=840

Answer 2

an = a1+ (n-1).r

Sn=(a1+an).n/2

a20 = 4+(20-1).4
a20 = 4+(19).4
a20 =4+76
a20=80

S20=(4+80)*20/2
S20 = (84)*20/2
S20 =84*10
S20 = 840