Question

Determine a soma dos 30 termos da sequência (3, 6, 9, 12)

Answer 1

3+6+9+12+15+18+21+24+27+30+33+36+39+42+45+48+51+54+57+60+63+66+69+72+75+78+81+84+87+90= 1395

Answer 2

$a1 = 3 \\ r = 6 - 3 \: -> 3 \\ n = 30 \\ a30 = ?$

$an = a1 + (n - 1) \times r$

$a30 = 3 + (3 0 - 1) \times 3 \\ a30 = 3 + 29 \times 3 \\ a30 = 3 + 87 \\ a30 = 90 < - -$

Agora a soma

$sn = \frac{(a1 + an) \times n}{2}$

$sn = \frac{(3 + 90) \times 30}{2}$

$sn = \frac{93 \times 30}{2}$

$sn = \frac{2790}{2}$

$sn = 1395 < - - -$

1395 é a soma.