One electron collides elastically with a second electron initially at rest. After the collision, the radii of their trajectories are r1 and r2.

One electron collides elastically with a second electron initially at rest. After the collision, the radii of their trajectories are r1 and r2. The trajectories are perpendicular to a uniform magnetic field of magnitude B. Determine the energy of the incident electron. (Use any variable or symbol stated above along with the following as necessary: me for the mass of elecron and e for the electron's charge.)