GeoOrArith.hs

main = interact $ show . f . map read . words

f (n:a:b:c:_)
  | b - a == c - b = arith !! n
  | otherwise      = geo !! n
  where
    arith = [a, b ..]
    geo   = if diffsigns || increasing
              then a:map (*d) geo
              else a:map (`div` e) geo
      where
        diffsigns  = signum a /= signum b
        d          = c `div` b
        e          = a `div` b
        increasing = b > a