Final answer:
In the equation tc = f + vx, 'v' is best described as the variable representing velocity or speed.
Step-by-step explanation:
In the cost equation tc = f + vx, the best description of 'v' is the variable velocity. In various physics equations, 'v' typically represents velocity or speed, such as in the equation v = vo + at, which shows that v (final velocity) equals vo (initial velocity) plus acceleration (a) times time (t). It's important to note that 'v' could represent different forms of velocity, such as average velocity in the equation V = (Vo+v) / 2 when acceleration is constant, or even the specific velocity component in the context of vector components.