Write a function to find t-nth term of geometric series.
import math
def tn_gp(a,n,r):
  tn = a * (math.pow(r, n - 1))
  return tn
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find t-nth term of geometric series.
import math
def tn_gp(a,n,r):
  tn = a * (math.pow(r, n - 1))
  return tn[/INST]assert tn_gp(1,5,2)==16
assert tn_gp(1,5,4)==256
assert tn_gp(2,6,3)==486</s>