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Homework Help: Maths c

  1. Sep 26, 2005 #1
    could some1 help me with the working of this question. it's for homework and i would help if i knew how to come to the answer in the back of the book.

    it says to find the general solution (y=...)

    given y'(x) = 2/(7-4y)

    ...


    how do u get to this:
    y = 7/4 +/- √(k-x)

    that is: seven over four plus or minus the squareroot of k minus x

    k being another constant

    please help if u can

    thanx
     
  2. jcsd
  3. Sep 26, 2005 #2

    HallsofIvy

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    Science Advisor

    Surely your text book (not to mention your teacher!) has talked about "separating variables"!

    y'= 2/(7- 4y) is the same as [itex]\frac{dy}{dx}= \frac{2}{7-4y}[/itex]

    "Separate" that into differential form with y on one side of the equation and x on the other:
    (7- 4y)dy= 2dx.
    Integrating: 7y-2y2= 2x+ k.

    Now think of that as the quadratic equation 2y2- 7y+ (2x+k)= 0 and solve for y using the quadratic formula. (The "C" in Ay2+ By+ C is 2x+k.)
     
  4. Sep 26, 2005 #3
    thank you very much

    this is a great forum

    very responsive
     
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