Question

calcule a soma dos 40 primeiros termos da progressao aritmatica (P.A.) (4,6,8,...)

Answer 1

a1=4
a2=6

an=a1+(n-1)r
6=4+(2-1).r
6=4+1r
r=6-4
r=2

a40=?

a40=4 + (40-1).2
a40=4 + 39.2
a40=4+78
a40=82

Sn= (a1+an).n/2
S40= (a1+a40).40/2
S40= (2+40).40/2
S40=42.40/2
S40= 1680/2
S40= 840