Fiindca este o progresie aritmetica poti folosii formula mediei aritmetice
[tex] \frac{a1 + a3}{2} =a2 [/tex]
[tex] \frac{2 + 8}{2} = 5[/tex]
[tex]a2 = 5[/tex]
[tex]a2 = a1 + r \: = 2 + r[/tex]
[tex]5 = 2 + r = > r = 3[/tex]
[tex]a10 = a1 + 9r = 2 + 9 \times 3 = 29[/tex]
[tex]s = \: \frac{n(a1 + a10)}{2} = \frac{10(2 + 29)}{2} = \frac{310}{2} = 155[/tex]
Formulele aplicate sunt:
[tex]a(n) = \frac{a(n - 1) + a(n + 1)}{2} [/tex]
(aici am pus paranteze in loc de indice)
[tex]s(n) = \frac{n(a1 + a(n))}{2} [/tex]
R:155
