func series (n) {
    var (u, v) = (2, -4)
    (n-2).times { (u, v) = (v, 111 - 1130/v + 3000/(v * u)) }
    return v
}

[(3..8)..., 20, 30, 50, 100].each {|n|
    printf("n = %3d -> %s\n", n, series(n))
}