问题描述:
Write a function to find t-nth term of geometric series.

测试用例:
['assert tn_gp(1,5,2)==16', 'assert tn_gp(1,5,4)==256', 'assert tn_gp(2,6,3)==486']

生成的代码:
def tn_gp(a, r, n):
    return a * (r ** (n - 1))

# Test cases
assert tn_gp(1, 5, 2) == 16
assert tn_gp(1, 5, 4) == 256
assert tn_gp(2, 6, 3) == 486

#Failure Cause
对测试用例理解错误，导致参数设计错误