**1. The problem statement, all variables and given/known data**

A gammaphoton with a frequency of 5,49*10^20 Hz passes by near a nucleus, and turns into a electron/positron pair. The electron has an kinetic energy of 1,20*10^-13 J. How much kinetic energi is in the posistron?(we can neglect the energyexchange with the nucleus)

**3. The attempt at a solution**

Ive tried to solve this using this formula but i get a negative answer:

hf=2E+Ekp+Eke => Ekp=hf-2E-Eke

Where Ekp is the kinetic energi of the positron, "hf" is the energy of the photon, "E" is the rest energy of the electron and "Eke" is the kinetic energy of the electron.

I get that Ekp=-1,64*10^-10 J. This isnt right?

http://ift.tt/1hEP18t