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 in still water is v . The boat is to make rtxn 7n8 (k.eta roun 3hk8uxxs+/i7 sf(0eb ze2mmsd 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 make2hmefbui xss7xs3+m z ke80 (/s 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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