I have never had a tattoo, but if did, it would be:

What this means is, your life is the sum of all happiness you accumulate over the course of your lifetime. For some, they would need to replace Death with the Time you are Forgotten.

I used this equation to teach my 12 year old nephew calculus. I love this equation, because I believe that everyone has a different definition for happiness. E.g.

Happiness = (Joy shared with friends & family) ^2 + Life accomplishments + Time spent travelling

What is your definition of happiness?