This is a great investigation from N-Rich which combines geometry with simple algebra. Your task is to find a rule that will calculate how many lines there are in any mystic rose. To construct a Mystic Rose draw a circle then use your protractor to place equally spaced dots around the circle. You then join each dot to every other dot with a straight line.
You can generate your own Mystic Roses using a spreadsheet on the bodmas blog.
Here’s another great starter from Transum