Particle in a uniform magnetic field

[bowdish.9]

Homework Statement

A particle with charge q and kinetic energy K travels in a uniform magnetic field of magnitude B. If the particle moves in a circular path of radius R, find expressions for speed, v, and mass, m.. (Use any variable or symbol stated above as necessary.)

Homework Equations

K=1/2mv^2 F=qvB=(mv^2)/R

The Attempt at a Solution

2K=mv^2 ---> qvB=(2K)/R ---> (2K)/(qBR)

Why is this incorrect?

 

[mfb]

If this is supposed to be the velocity, it looks correct (if we do not include special relativity).
Why do you think it is incorrect?