Imagine that you go running with a friend. You run at 0.9 * speed of light and your friend runs at 0.8 * speed of light in the exact opposite direction. How fast are you getting away from each other?
I'm not sure I understand. It seems like you're implying that the two people would be moving away from each other faster than the speed of light, but I was under the impression that that's not the case. I thought that each person would observe the other travelling at C?
1.7c for an observer just standing around and watching the two guys run. I am not sure what it looks like from the point of view of the runners without calculating it but they should see a speed smaller than the speed of light.