Write a function to find the nth octagonal number.
def is_octagonal(n): 
	return 3 * n * n - 2 * n 
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the nth octagonal number.
def is_octagonal(n): 
	return 3 * n * n - 2 * n [/INST]assert is_octagonal(5) == 65
assert is_octagonal(10) == 280
assert is_octagonal(15) == 645</s>