input: Write a python function to calculate the area of a trapezoid.
  assert area_trapezoid(2, 4, 3) == 9.0
  assert area_trapezoid(5, 7, 10) == 60.0
  assert area_trapezoid(1, 1, 1) == 1.0
output: def area_trapezoid(base1, base2, height):
      return 0.5 * (base1 + base2) * height