input: Write a function to calculate the sum of an exponential series for a given base `x` and number of terms `n`. The function should return the sum of the series.
  
  def exponential_series(x, n):
      sum = 0
      for i in range(n+1):
          sum += x ** i
      return sum
  
  assert exponential_series(2, 4) == 31
  assert exponential_series(3, 3) == 40
  assert exponential_series(1, 10) == 11
  assert exponential_series(0, 5) == 1
  assert exponential_series(5, 0) == 1
output: def exponential_series(x, n):
      sum = 0
      for i in range(n+1):
          sum += x ** i
      return sum