A 0' B 比 -u S系 A 0 B -x t+ u XA tr=0,x4<0→t4<0 2 c3 u t8+ XB ,tg=0,xg>0→tg>0, 1 2 2 2 c u x c u t t A A A − ′ + ′ = ′ ′ , 1 2 2 2 c u x c u t t B B B − ′ + ′ = ′ ′ < 0 A ′ = , 0 t A′ t x ′ A′ < 0 ′ ′ = , 0 B t > 0 > 0 B ′ t x B′ A O B A′ O′ B′ x x′ u r S ′系