It's my birthday today. Hurray! On the table this morning are some presents wrapped in mathematical wrapping paper. I haven't opened them yet, but I did enjoy identifying the formulas.
Here's a quiz for you: One of the formulas has an error. That is, one of the formulas is supposed to be a well-known formula, but it's written incorrectly so it's some other formula. Can you find it?
Leave your answer in the comments!