Another approach to fibonacci in Ruby

integer.rb

class Integer
  
  # PHI - Some greek number
  PHI   =  Rational(1836311903,1134903170).to_f

  # Inverse of 1/PHI
  I_PHI = -Rational(1, PHI).to_f

  # Square root of 5
  SQRT_5 = 5 ** 0.5
    
  def fibonacci
    raise ArgumentError, "Can't return fibonacci value for integer < 0" if self < 0
    return ((PHI**self - I_PHI**self) / SQRT_5).round
  end
  
end

puts 100.fibonacci