Learning Ruby: Knowing.net Exercise #1

I've just finished exercise #1 from: knowing.net. Please feel free to critique this code, I am serious about learning Ruby and getting introduced to members of the community.

proj1.rb

# Write a program that takes as its arguments one of the words ’sum’,
# ‘product’, ‘mean’, or ’sqrt’ followed by a series of numbers. The
# program applies the appropriate function to the series.

# the mathn library makes the Math.sqrt call below behave better in
# situations involving negative sums.
require 'mathn'

class Array
  def sum
    # missed the inject method on my first
    # skim of the 'Programming Ruby' appendix.
    self.inject {|sum, i| sum + i}.to_f
  end
  def product
    self.inject(1) {|product, i| product * i}.to_f
  end
  def mean
    self.sum / self.size
  end
  def sqrt
    Math.sqrt(self.sum)
  end
end

raise "usage: ruby proj1.rb {sum|product|mean|sqrt} {value}+" if ARGV.size < 2

Allowed_commands = ["sum", "product", "mean", "sqrt"]
command = ARGV.shift.downcase
raise "unknown command: #{command}" unless Allowed_commands.include? command

# I don't like this line, is there a better
# way to do String -> numeric conversions?
ARGV.collect! {|i| i.to_f}
puts eval ARGV.to_s + "." + command