t Ox 为此需要:0 t x an an 故可令: x=a(2+m) E=(x+ at)/2a In=(xr-at)/2aïïîïïíì = ¶¶ = ¶¶ 11 hxtt ïïîïïíì = - ¶¶ = ¶¶ a x a xh 为此需要： x 故可令： îíì = - = + x hx h tx a ( ) îíì = - = + x at a x at a ( ) / 2 ( ) / 2 h 即： x