What should I do if I have just observed minor mathematical typos in my accepted paper?
As you wrote yourself:


  The editor has told me that the paper has been sent already for printing, and nothing can be done anymore.


Given that the editor decides how a publication runs, there is nothing you can do. "Normal" journals usually publish errata but conference papers often do not have that possibility.

Don't worry about it too much: everybody makes mistakes/typos and it is not a big problem. The intelligent reader will spot the mistakes and not mind too much because he gets the point anyway, the other readers will not spot the mistake so they won't care either. 

Just make sure you learn from this: next time double check everything and ask a meticulous, friendly colleague to do the same.