Resposta :
an=?
a1=1
n=38
r=3
an=a1+(n-1)*r
an=1+(38-1)*3
an=1+(37)*3
an=1+111
an=112
Sn=[(a1+an)*n]/2
Sn=[(1+112)*38]/2
Sn=[113*18]/2
Sn=4294/2
Sn=2147
***Resposta: Sn=2147
a1=1
n=38
r=3
an=a1+(n-1)*r
an=1+(38-1)*3
an=1+(37)*3
an=1+111
an=112
Sn=[(a1+an)*n]/2
Sn=[(1+112)*38]/2
Sn=[113*18]/2
Sn=4294/2
Sn=2147
***Resposta: Sn=2147
Me parece ser uma P.A. razão 3. Então os 38 primeiros termos serão
P.A. (1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 73, 76, 79, 82, 85, 88, 91, 94, 97, 100, 103, 106, 109, 112)
E a soma dos termos é 2.147
P.A. (1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 73, 76, 79, 82, 85, 88, 91, 94, 97, 100, 103, 106, 109, 112)
E a soma dos termos é 2.147