1.

A boat which has a speed of 5 km per hour in still water crosses a river of width 1 km along the shortest possible path in fifteen minutes. The velocity of the river water in km per hour is :-

Answer»

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`sqrt(41)`

Solution :
`vecV_(br)=vecV_(b)-vecV_(R)`
`V_(br)^(2)=V_(b)^(2)+V_(r)^(2)`
`V_(b)^(2)=25-V_(r)^(2) "" ...(1)`
`t=("distance")/(V_(b))rArr `
`(1)/(4) HR = (1)/(sqrt(25-V_(r)^(2)))`
`V_(r)=3 km//h`


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