1节=1kn=1海里/小时
设：航行的时间x小时,甲.乙两船的距离为y海里.
y=√{[cos60°·8x+(20-10x)]^2+(sin60°·8x)^2}
y=√{[4x+(20-10x)]^2+(4x√3)^2}
y=√{[20-6x]^2+(48x^2)}
y=√（400-240x+36x^2+48x^2)
y=√（400-240x+84x^2)
y=2√（100-240x+21x^2)
y=2√[21·（x-30/21)^2+400/7]
所以当x=30/21小时时,甲.乙两船的距离最近,且为（40√7）/7海里.