### what is the "....;"?

I have just installed the SAGE and tested it with some equations. But I found a problem that the left side of the command line changes from "sage: " to "....;". It was the case when I typed the following codes.

f(x)=2.4/x^2*(1-e^(-x/50))

Could anyone let me know what does "...;" means? andy how to recover the normal command line in order to use the SAGe again? with the ...; it seems that the SAGe never work.

thank you.