There is a code block that shouldn't be here. I'm guessing the `'` after n(n+1) should be a `` ` ``. <img width="1060" alt="Screenshot 2020-10-01 at 17 20 57" src="https://user-images.githubusercontent.com/33573717/94829141-81df2100-040a-11eb-8cbf-19b1e48dcf66.png">