回复：回复：我请教一个高小的题吧。

swimming along the 30 degree angle,1m/s * tan(30) = 0.5m/s.  This will keep the dog always smim straight forward toward opposite B point.  The net speed to swim cross the river is 1m/s * tan(60) = 0.86605 m/s.  Rive width 20m: t = 20/0.86605 = 23s

 

 

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• 回复：Agree. Net speed of dog and river flow is sqrt(3)/2 -Yangtze430030- ♂ (0 bytes) () 02/04/2011 postreply 09:23:01