Question

Obtenha a soma dos termos da P.A (100,95,...20) e marque a resposta correta a)1000 b)1020 c)1040 d)1080

Answer 1

sn = (a1 + an) *n /2

sn= (100 + 20) * n /2

n = ?

an = a1 + (n-1) *r

20 = 100 + (n - 1) *(-5)

20 = 100  - 5n + 5

20 = 105 - 5n

20 - 105 = - 5n

-85 = - 5n

- 85 / - 5 = n

17 = n

voltando

sn= (100 + 20) * 17 /2

sn = (120 * 17) /2

sn = 2040 /2

sn = 1020