Write a function to find the surface area of a sphere.
import math
def surfacearea_sphere(r):
  surfacearea=4*math.pi*r*r
  return surfacearea
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the surface area of a sphere.
import math
def surfacearea_sphere(r):
  surfacearea=4*math.pi*r*r
  return surfacearea[/INST]assert surfacearea_sphere(10)==1256.6370614359173
assert surfacearea_sphere(15)==2827.4333882308138
assert surfacearea_sphere(20)==5026.548245743669</s>