Write a function to find the volume of a sphere.
import math
def volume_sphere(r):
  volume=(4/3)*math.pi*r*r*r
  return volume
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the volume of a sphere.
import math
def volume_sphere(r):
  volume=(4/3)*math.pi*r*r*r
  return volume[/INST]assert volume_sphere(10)==4188.790204786391
assert volume_sphere(25)==65449.84694978735
assert volume_sphere(20)==33510.32163829113</s>