本题目来源于试卷: gc physics textbook chapter 3 problems，类别为 IB数学
[问答题]
The speed of a boat -6th0phwf-pxh( fw (okszl*2f35 cxlin stilbtg+z y93b(y el water is v . The boat is to make a round trip in a river whose current travelsyye+9 t (zbgb3 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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