The magnetic field is approximately K B at the corner(ao, zo)of the dashed lines, it is simply B=(roi+ zok The velocity of the proton at(e0, 2o) is given by Vo=v00j+002k Note that here 000<0, and v0z <0 The Lorentz force acting on the proton is K F=e(v B)=e(voej+v02k)x(aoi+ zok)3