document.write( "Question 445844: A boat travels 10 km upstream and 10 km back. The time for the round trip is 10 hours. The speed of the stream is 4 km/hr. What is the speed of the boat in still water? \n" ); document.write( "

Algebra.Com's Answer #307054 by mananth(16946) $\"\"$ $\"About$

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Boat speed x
\n" ); document.write( "current speed 4
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\n" ); document.write( "against current x-4 mph
\n" ); document.write( "with current x+ 4 mph
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\n" ); document.write( "Distance= 10 miles
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\n" ); document.write( "Time agianst + time with = 10 hours
\n" ); document.write( "t=d/r
\n" ); document.write( "10/(x +4)+10/(x -4)=10
\n" ); document.write( "LCD =(x -4)(x+4)
\n" ); document.write( "10*(x-4)+10x+4)=10
\n" ); document.write( "10x-40+10x+40=10(x ^2-16)
\n" ); document.write( "20x=10x^2-160
\n" ); document.write( "10x ^2-20x-160 =0
\n" ); document.write( "Find the roots of the equation by quadratic formula
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\n" ); document.write( "a= 10 b= -20 c= -160
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\n" ); document.write( "b^2-4ac=400+6400
\n" ); document.write( "b^2-4ac=6800
\n" ); document.write( "sqrt(6800)=82.46
\n" ); document.write( " $\"x=%28-b%2B-sqrt%28b%5E2-4ac%29%29%2F%282a%29\"$
\n" ); document.write( " $\"x1=%28-b%2Bsqrt%28b%5E2-4ac%29%29%2F%282a%29\"$
\n" ); document.write( "x1=(20+82.46)/20
\n" ); document.write( "x1=5.12
\n" ); document.write( "x2=(20-82.46)/20
\n" ); document.write( "x2= -3.12
\n" ); document.write( "Ignore negative value
\n" ); document.write( " speed = 5.12 mph
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