Question

Numa p.a sabemos que a1=1, an=x, r=6, e sn=280, determine o valor de x.

Answer 1

280 = (1 + an) . n/2
560/n -1 = an

560/n  -1 = 1 +(n -1) . 6
560/n -2 = n6 -6
560 +4n = 6n²
3n² -2n -280 = 0

Δ = -2² -4 . 3 . -280
Δ = 4 + 3360
Δ = 3364

n = [-(-2) ± √3364]/(2 . 3)

n' = 10
n'' = 28/3

n ∈ N*

560/10 -1 = an
55 = an