maths
2008-03-29 5:19 am
a ship take 5 minutes more to sail a journey of 4.5 km against the current than it takes to sail with the current(assume constant throughout).if the speed of the ship is still 8m/s. find the speed of the current
回答 (2)
2008-03-29 5:27 am
✔ 最佳答案
Let the speed of the current be xm/s.5min = 300s
When it sails against the current, the time taken:
4500 / ( 8 - x )
When it sails with the current, the time taken:
4500 / ( 8 + x )
As given,
4500 / ( 8 - x ) - 4500 / ( 8 + x ) = 300
15 ( 8 + x ) - 15 ( 8 - x ) = ( 8 - x )( 8 + x )
15 ( 2x ) = 64 - x2
x2 + 30x - 64 = 0
( x + 32 )( x - 2 ) = 0
x = -32 ( rejected ) or x = 2
Hence the speed of the current is 2m/s.
參考: My Maths Knowledge
2008-03-29 5:36 am
let x the the speed of the current.
4500/(8-x) - 4500/(8+x)=5x60
15/(8-x) - 15/(8+x)=1
15(8+x)-15(8-x)=(8-x)(8+x)
30x=64-x^2
x^2+30x-64=0
(x+32)(x-2)=0
x=2, x=-32rejected
the speed of the current is 2m/s
4500/(8-x) - 4500/(8+x)=5x60
15/(8-x) - 15/(8+x)=1
15(8+x)-15(8-x)=(8-x)(8+x)
30x=64-x^2
x^2+30x-64=0
(x+32)(x-2)=0
x=2, x=-32rejected
the speed of the current is 2m/s
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