Question

Determine a soma dos 18 primeiros termos da PA (1 , 4 , 7 , .....)

Answer 1

a1=1
razão=1-4=3
n=18


an=a1+(n-1).r
a18=1+(18-1).3
a18=1+17.3
a18=1+51
a18=52


sn=(a1+a18).n/2
s18=(1+52).18/2
s18=53.18/2
s18=954/2
s18=477