Question 375325
let speed of boat in still water be x mph
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current speed = 6 mph.
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speed upstream = x-6 mph
speed downstream = x+6 mph.
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t = d/r
time upstream + time downstream = 4 hours
{{{80/(x-6)+80/(x+6) =4}}}
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{{{(80*(x+6)+80*(x-60))/((x+6)*(x-6))=4}}}
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{{{(80x+480+80x-480)/((x+6)(x-6))=4}}}
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{{{(160x)/((x+6)(x-6))=4}}}
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multiply by (x+6)(x-6)
{{{(160x)*(cross((x+6)(x-6)))/((cross((x+6)(x-6))))=4(x+6)(x-6)}}}
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160x= 4x^2-36
4x^2-160x-36=0
/4
x^2-40x-9=0
discriminant = 1636
x1=((40+sqrt(1636)))/2
x1=40.2 mph speed of boat in still water
x2 will be negative so ignore
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m.ananth@hotmail.ca