Proof that time <0 for tachyons > c

[Powergade]

1. Homework Statement

Derive the result DeltaT <0 for U> (sqrt(1-v^2/c^2)+1)/v/c)c

Homework Equations

DeltaT = u/l + (u-v/1-uv/c^2)1/l

Where:
DeltaT is the time for the tachyon to go and come back.
u is the velocity of the tachyon
l is the distance that the tachyon goes
v is the velocity of the receiving end moving away from the tachyon
c is the speed of light

The Attempt at a Solution

[/B]
I tried to substitute values for u(2c),l(100m) and v(0.5c) but it gives me DeltaT = something/0 and I'm trying to get time less than zero, not infinite time.

I'm not sure that I need to put values in the equations to proof that DeltaT <0 for U> (sqrt(1-v^2/c^2)+1)/v/c)c .

 

[Algr]

If you get time less than zero, doesn't that mean you have turned the tachyon into antimatter? Or does matter/antimatter not apply to tachyons?