本题目来源于试卷: gc physics textbook chapter 3 problems，类别为 IB物理学
[问答题]
The speed of a boat in still water is v . The boat is t:h4 x7egdh;xhf6+in-qi wcvs l53y9 ko make a52ypu9pmajz 5 xbz atq;y8 6t1 round trip in a river whose current travels at speed u . Derive a formula for the time needed to make a round trip of total distance D if the boat makes the round trip by moving (a) upsbmpt;2j 6 u xtpz95aq1y8ya5 ztream and back downstream, (b) directly across the river and back. We must assume u
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