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## Homework Statement

Two spaceships are traveling on the interstellar freeway. Ship A is traveling at 0.5 c and Ship B is traveling at 0.8 c. Ship B passes Ship A. How fast does the pilot of Ship A see Ship B moving away from her? (c is speed of light)

## Homework Equations

I think the velocity-addition formula is relevant.

V = (v1 + v2) / (1 + ((v1/c) x (v2/c)))

## The Attempt at a Solution

V = (0.5c + 0.8c) / (1 + ((0.5c/c) x (0.8c/c))) = 0.93c

This answer just doesn't make sense to me which leads me to believe that one of the values should be negative, or I need to alter the formula.

I'm wondering if maybe it should be V = (0.8c - 0.5c) / (1 + ((0.8c/c) x (0.5c/c))) = 0.21c. I really don't know.