O' B 比 S系 X B u 、XA 2,=0,x<0→t>0 1 2 u c ,tB=0,xg>0→tg<0 u A O B A′ O′ B′ x x ′ u r , 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 At <0 Ax ′ ′ < 0 Bt =0, Bt >0 Bx S 系