10. It’s Fermat’s birthday

9. I didn’t know whether “i” is the square root of -1 or “i” are the square root of -1.

8. I accidentally divided by 0 and my paper burst into flames.

7. I had too much pi and got sick.

6. I could only get arbitrarily close to my textbook.

5.

It’s stuck inside a Klein bottle.

4. Someone already published it, so I didn’t bother to write it.

3. My 4-dimensional dog ate it.

2. I have a solar calculator and it was cloudy yesterday.

1. There wasn’t enough room to write it in the margin.