Write a function to find the n-th rectangular number.
def find_rect_num(n):
  return n*(n + 1) 
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the n-th rectangular number.
def find_rect_num(n):
  return n*(n + 1) [/INST]assert find_rect_num(4) == 20
assert find_rect_num(5) == 30
assert find_rect_num(6) == 42</s>