Write a function to find the nth decagonal number.
def is_num_decagonal(n): 
	return 4 * n * n - 3 * n 
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the nth decagonal number.
def is_num_decagonal(n): 
	return 4 * n * n - 3 * n [/INST]assert is_num_decagonal(3) == 27
assert is_num_decagonal(7) == 175
assert is_num_decagonal(10) == 370</s>