My Hand-Made ruby cosine function

cos.rb

def cos(num,its=1000)
	def factorial(num)
		return 1 if num == 0
		num.downto(1).inject(:*)
	end
	sum = 0.0
	sign = 1
	its.times do |i|
		exp = i*2
		sum += sign * num**exp / factorial(exp)
		sign *= -1.0
	end
	sum
end