1. The problem statement, all variables and given/known data what is the speed of an electron with kinetic energy of (a) 100000 eV 2. Relevant equations m=m0/√(1-β2) K=Δmc2 3. The attempt at a solution plug first into second equation, isolate v and it eventually comes out to be v=c(1-(1+K/m0c2)-2) plug in numbers and i get a velocity of 0.9*108 m/s the answer is 1.6*108 m/s in the textbook. Note: This isn't a school assignment. I am self studying for physics olympiad, and this is my first encounter with relativity from a mathematical stand point.