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Sagot :
the speed of the boat is 5.418 km/h and the speed of the stream is 2.167 km/h
Let's define the variables:
B = speed of the boat in still water.
S = speed of the stream.
When the boat travels downstream, the total speed of the boat will be equal to the sum of the speed of the stream and the speed of the boat in still water:
speed = B + S
When the boat goes downstream, the speed will be:
speed = B - S
Now, also remember the relation:
speed*time = distance.
The given information is:
In 8 hours the boat can:
go 12 km upstream and 40km downstream.
We now need to define another variable, T, as the time that the boat travels upstream.
Then we can write this as:
12km = (B - S)*T
If the boat travels T hours upstream, and travels for a total of 8 hours, then the amount of time that travels downstream is 8h - T, then we can write:
40km = (S + B)*(8h - T)
Similarly, when we have:
"it can go 16 km upstream and 32 km downstream in the same time."
we can define a new variable T', and write:
16km = (B - S)*T'
32km = (S + B)*(8h - T')
Then we have a system of 4 equations:
16km = (B - S)*T'
32km = (S + B)*(8h - T')
40km = (S + B)*(8h - T)
12km = (B - S)*T
And we need to solve this for S and B.
To do it, we need to isolate one of the variables in one of the equations.
Let's isolate T in the last equation:
T = 12km/(B - S)
now we can replace that in the third equation to get:
40km = (S + B)*(8h - 12km/(B - S))
So now we have 3 equations:
16km = (B - S)*T'
32km = (S + B)*(8h - T')
40km = (S + B)*(8h - 12km/(S - B))
Now we need to do the same thing, this time let's isolate T' in the first equation and replace it in the second one:
T' = 16km/(B - S)
Replacing it in the second equation we get:
32km = (S + B)*(8h - T')
32km = (S + B)*(8h - 16km/(B - S))
So now we have two equations:
40km = (S + B)*(8h - 12km/(B - S))
32km = (S + B)*(8h - 16km/(B - S))
Let's simplify these:
40km = 8h*(S + B) - 12km*(S + B)/(B - S)
32km = 8h*(S + B) - 16km*(S+ B)/(B - S)
Now we can multiply both equations by (B - S) to get:
40km*(S - B) = 8h*(S + B)*(B - S) - 12km*(S + B)
32km*(S - B) = 8h*(S + B)*(B - S) - 16km*(S+ B)
Let's keep simplifying this:
40km*(B - S) + 12km*(S + B) = 8h*(S + B)*(B - S)
32km*(B - S) + 16km*(S+ B) = 8h*(S + B)*(B - S)
Now we get:
52km*B - 28km*S = 8h*(S^2 + B^2)
48km*B - 16km*S = 8h*(S^2 + B^2)
Notice that the right side of these equations is the same thing, then we can write:
52km*B - 28km*S = 48km*B - 16km*S
(52km - 48km)*B = (28km - 18km)*S
4km*B = 10km*S
B = (10/4)*S
B = (5/2)*S
Now we can replace this in one of our two equations, let's use the first one:
48km*B - 16km*S = 8h*(S^2 + B^2)
48km*(5/2)*S - 16km*S = 8h*( S^2 + ( (5/2)*S)^2)
Now we can solve this for S
104km*S = 8h*( S^2 + 25/4*S^2)
104km*S = 8h*(29/4*S^2) = 48h*S^2
104km*S = 48h*S^2
dividing at both sides by S we get:
104km = 48h*S
104km/48h = S = 2.167 km/h
And using B = (5/2)*S
We can find the speed of the boat:
B = (5/2)*2.167 km/h = 5.418 km/h
Then:
the speed of the boat is 5.418 km/h and the speed of the stream is 2.167 km/h
If you want to learn more about this topic, you can read:
https://brainly.com/question/17300107
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