Question

Cosideram sirul de nr:2,8,14,20,26... c)calculează suma primilor 50 de termeni ai sirului

Answer 1

Fie sirul 2, 8, 14, 20, 26...
a1 = 2
r = a2-a1 = 8-2 = 6

an = a1 + (n-1)*r
a50= 2 + 49*6 = 2 + 294 = 246

$sn = \frac{(a1 + an) \times n}{2}$
$sn = \frac{(2 + a50) \times50}{2}$
$sn = (2 + 296) \times 25$
Sn = 298 × 25 = 7450

Answer 2

2,8,14,20,26…..

a1=2; a2=8 => r=6

an=a1+(n-1)·r

Sn=n(a1+an)/2

a50=2+49·6

a50=296

S50=50(2+296)/2

S50=25·298

S50=7450