Problem 7 on Project Euler

p7.rb

class Numeric
  def divisible_by? num
    self % num == 0
  end

  def prime?
    not (2..self.sqrt.floor).any? {|i| self.divisible_by? i}
  end

  def sqrt
    Math.sqrt self
  end
end

primes = [2, 3]
until primes.size == 10001
  i = primes.last + 2
  i += 2 until i.prime?
  primes << i
end
puts primes.last