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 sd77dzh8lx 5sg 1 t gr;om b:s)(ckbau9till water is v . The boat is to make a round trip in a river whose current travels(bco sm u:a)bg ;k1tr9 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) upstream and back downstream, (b) directly across the river and back. We must assume u
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