SOLUTION: speed of a boat in still wateris 9km/hr.It goes 12km down stream and comes back to the starting point in 3hrs.What is the speed of water in the stream?

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Question 388616: speed of a boat in still wateris 9km/hr.It goes 12km down stream and comes back to the starting point in 3hrs.What is the speed of water in the stream?
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
speed of a boat in still wateris 9km/hr.It goes 12km down stream and comes back to the starting point in 3hrs.What is the speed of water in the stream?
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boat speed = 9 km/h
speed of current = x km/h
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upstream speed = 9-x
downstream speed = 9+x
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time downstream + time upstream = 3 hours
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t=d/r
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12%2F%289-x%29%2B12%2F%289%2Bx%29=3
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LCD = (9+x)(9-x)
multiply by LCD
%2812%289%2Bx%29-12%289-x%29%29%2F%28%289%2Bx%29%289-x%29%29=3
(108+12x-108+12x)=3(9+x)(9-x)
24x=3(81-x^2)
/3
8x=81-x^2
x^2+8x-81=0
find roots x1,x2 by using quadratic formula
discriminant = b^2-4ac = 388
x1=%28-8%2Bsqrt%28388%29%29%2F%282%29
%28highlight%28x1=5.85%29%29 km/h speed of current
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x2 will be negative so ignore
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m.ananth@hotmail.ca