import math

def cos(x, n):
  formula = lambda k: (-1)**k * x**(2*k) / math.factorial(2*k)
  soma = 0

  for k in range(n+1):
    soma += formula(k)

  return soma

assert math.isclose(cos(0.5, 10), math.cos(0.5))
assert math.isclose(cos(-0.5, 10), math.cos(-0.5))
assert math.isclose(cos(0.0, 10), math.cos(0.0))
assert math.isclose(cos(1.0, 10), math.cos(1.0))