                 30   \               20   \                  8—3           30   \            20    \                       30＋20＝50     \             8—3      
                 8—4      B    A    2×108  \        B                  3×108  \       B        3×108  \             B                          8—4    B    A   2×108  \                     3×108  \     
                A                 A        3×108 ＋  2×108 ＝5×108  \                            3×108   \    
                                  A     3×108 m \ s                                         
                  50   \      49.999999999999933   \                                                                                       c      
                                                                                                                                                
                                                                                                                    