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?

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) (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?
...
boat speed = 9 km/h
speed of current = x km/h
...
upstream speed = 9-x
downstream speed = 9+x
..
time downstream + time upstream = 3 hours
..
t=d/r
..

...
LCD = (9+x)(9-x)
multiply by LCD
=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

km/h speed of current
..
x2 will be negative so ignore
...
m.ananth@hotmail.ca
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