input: Write a function to calculate the sum of an exponential series for a given value of x and the number of terms n.
  def exponential_series(x,n):
      sum = 0
      for i in range(n+1):
          sum += x ** i
      return sum
  
  assert exponential_series(2,3)==15
  assert exponential_series(3,4)==121
  assert exponential_series(0,5)==6
  assert exponential_series(1,100)==101
  assert exponential_series(-1,5)==0
output: def exponential_series(x, n):
      sum = 0
      for i in range(n + 1):
          sum += x ** i
      return sum