Fibonacci sequence in ruby

fibonacci.rb

def fib(n = 5)
  first_two = [0, 1]
  result = first_two + []
  n.times do |i| 
    ans = first_two.inject(:+)
    result.push(ans)
    first_two = [first_two.last, ans]
  end
  result.slice(0, n)
end 

fibonacci = Hash.new{ |h,k| h[k] = k < 2 ? k : h[k-1] + h[k-2] }