S.5續方程

設水流的速率 x km/h ,由城市A順流航行24km至另一城市B 需時 :24 / (10 + x) 小時再逆流駛回城市A 需時 :24 / (10 - x) 小時共需時5小時 :24 / (10 + x) + 24 / (10 - x) = 524 (10 - x) + 24 (10 + x) = 5(10 - x)(10 + x)480 = 5 (100 - x^2)5x^2 - 20 = 0x^2 - 4 = 0(x - 4)(x + 4) = 0x = 4 或 x = - 4 (捨去)水流的速率 = 4 km/h

2010-09-12 12:40:46 補充：
更正 :

x^2 - 4 = 0
(x - 2)(x + 2) = 0
x = 2 或 x = - 2 (捨去)
水流的速率 = 2 km/h