Write a function to find the vertex of a parabola.
def parabola_vertex(a, b, c): 
  vertex=(((-b / (2 * a)),(((4 * a * c) - (b * b)) / (4 * a))))
  return vertex
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the vertex of a parabola.
def parabola_vertex(a, b, c): 
  vertex=(((-b / (2 * a)),(((4 * a * c) - (b * b)) / (4 * a))))
  return vertex[/INST]assert parabola_vertex(5,3,2)==(-0.3, 1.55)
assert parabola_vertex(9,8,4)==(-0.4444444444444444, 2.2222222222222223)
assert parabola_vertex(2,4,6)==(-1.0, 4.0)</s>