input: Write a function to compute the area of a circle given its radius as input. If the radius is negative, return 0. Use the formula \( \text{area} = \pi \times r^2 \) where \( \pi \) is a constant (use math.pi) and r is the radius.
  import math
  assert area_circle(5) == 78.54
  assert area_circle(-5) == 0
  assert area_circle(0) == 0
output: def area_circle(r):
      if r < 0:
          return 0
      return math.pi * r ** 2