::: −1/c2 might give the wrong impression (that it cannot possibly be zero), which Delaszk had got  oh, *that* was the problem! I though it is obvious that 299792458 m/s is just as good as infinitely fast for all pedestrians on earth, and (a typical household velocity)^2/(299792458m/s)^2 is just as good as zero (unless you use atomic clock precision measurements). 

 ::: I think there is something wrong with what mathematicians teach their students about infinities and division by zero. Infinity is simply something much larger then (and zero is something much smaller then) the stuff you are concerned with. If you divide by something very small you get something very large. ) Why is it a taboo for mathematicians?