Question

Calcule a soma dos 20 primeiros termos da P.A. (7,15,23,...). ​

Answer 1

n=a1+(n-1).r

n=7+(20-1).8

n=7+19.8

n=7+152

n=159

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Sn=(a1+an).n/2

Sn=(7+159).20/2

Sn=166.20/2

Sn=3.320/2

Sn=1.660✓

Answer 2

Resposta:

1660

Explicação passo-a-passo:

an=a1+(n-1).r

an= 7+ (20-1).8

an= 7+ 19 . 8

an= 7+ 152

an= 159

a3=23

a4=31

a5=39

a6=47

a7=55......

sn=n:2× (a1+an)

sn=20:2 ×(7+159)

sn=20:2× (166)

sn=20 ×166=3320

sn=3320:2

sn=1.660