N A B, B,B, I BB AB1B,B2Ⅱ 时刻t 时刻t+△t A3B3=l1△t AB=L2△ ∠A,.AB,=i ∠BB,A=y sini A B 所以 sin r aB ui i i A1 A2 A3 B1 B2 B3 N N A I d Ⅰ Ⅱ Ⅰ Ⅱ 时刻 t 时刻 t+△t A B = u t 3 3 1 AB = u t 2 B1 B2 B3 N N A I r B r R r A AB = i 3 3 BB A = r 3 2 3 3 1 sin sin u u AB A B r i 所以 = =