input: Write a function to calculate the total cost of an item after tax.
  assert calculate_total_cost(5, 0.2) == 6.0
  assert calculate_total_cost(10, 0.1) == 11.0
  assert calculate_total_cost(100, 0.25) == 125.0
output: def calculate_total_cost(price, tax_rate):
      return price * (1 + tax_rate)