[填空题]
An observer on Earth sees an alien vessel approach at -4r0ac;7or4vpjat q ua spe o9h+0ki xwe9eed of 0.60c. The Enterprise comes to the rescue (Fig. Below), overtaking the aliens while moving directly toward Earth at a speed of 0.90c relative to Earth. What is the relative9k0+wox 9 ehie speed of one vessel as seen by the other? u = c
