Results 1 to 6 of 6

Thread: isolating a variable...difficulties

  1. September 13th 2009, 05:09 PM #1
    solarscott
    solarscott is offline
    Newbie
    Joined
    Sep 2009
    Posts
    10

    isolating a variable...difficulties

    Once again, you guys are helping me out heaps. Thanks! My maths teacher I believe has put many tricks into these questinos. I eventually need to find the gradient of this curve. I would normally isolate y, and then take the derivative. But I'm having issues isolating y. Any help is appreciated...thanks

    xy(x+y)+16=0
    Follow Math Help Forum on Facebook and Google+
  2. September 13th 2009, 05:18 PM #2
    skeeter
    skeeter is offline
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,625
    Thanks
    425
    Quote Originally Posted by solarscott View Post
    Once again, you guys are helping me out heaps. Thanks! My maths teacher I believe has put many tricks into these questinos. I eventually need to find the gradient of this curve. I would normally isolate y, and then take the derivative. But I'm having issues isolating y. Any help is appreciated...thanks

    xy(x+y)+16=0
    that is the main reason for using implicit differentiation.

    but, if you insist on isolating y ...

    xy^2 + x^2 y + 16 = 0

    y = \frac{-x^2 \pm \sqrt{x^4 - 64x}}{2x}
    Follow Math Help Forum on Facebook and Google+
  3. September 13th 2009, 05:21 PM #3
    solarscott
    solarscott is offline
    Newbie
    Joined
    Sep 2009
    Posts
    10
    ahhh, I knew there was another way to do it...just forgot the name..implicit differentiation! Thanks. I will look that up...Im believe that is the way he is expecting it to be done...
    Follow Math Help Forum on Facebook and Google+
  4. September 13th 2009, 05:21 PM #4
    Caturdayz
    Caturdayz is offline
    Newbie
    Joined
    Sep 2009
    Posts
    21
    It looks more like a FOIL problem to me...

    xy(x+y)+16=0

    x^2+y^2+2y+2x+16=0
    Follow Math Help Forum on Facebook and Google+
  5. September 13th 2009, 05:23 PM #5
    skeeter
    skeeter is offline
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,625
    Thanks
    425
    Quote Originally Posted by Caturdayz View Post
    It looks more like a FOIL problem to me...

    xy(x+y)+16=0

    x^2+y^2+2y+2x+16=0
    it's not a FOIL problem.
    Follow Math Help Forum on Facebook and Google+
  6. September 13th 2009, 06:09 PM #6
    solarscott
    solarscott is offline
    Newbie
    Joined
    Sep 2009
    Posts
    10
    Yep, I just worked it through to find what I was supposed to (the equation of the tangent where the gradient = -1, and the answer I think makes sense...

    I found:
    y'=(-y^2-2xy)/(x^2+2xy)

    subbing in y'=-1, I solve for:

    -1=(-y^2-2xy)/(x^2+2xy)
    -x^2-2xy=-2xy-y^2
    x^2=y^2
    y=-+x


    So the equation of the tangent where the gradient =-1 would be y=+-x? Does this make sense? Thanks
    Last edited by solarscott; September 13th 2009 at 06:28 PM. Reason: corrected mistake
    Follow Math Help Forum on Facebook and Google+
« Tell me im not a complete idiot :P | Finding thickness given width, length, mass, density »

Similar Math Help Forum Discussions

  1. isolating and solving for a variable
    Posted in the Algebra Forum
    Replies: 2
    Last Post: February 18th 2011, 12:55 AM
  2. Isolating a Variable
    Posted in the Pre-Calculus Forum
    Replies: 2
    Last Post: October 26th 2010, 04:12 PM
  3. Isolating a variable
    Posted in the Algebra Forum
    Replies: 9
    Last Post: October 10th 2010, 08:44 PM
  4. Isolating a Variable
    Posted in the Calculus Forum
    Replies: 7
    Last Post: April 12th 2010, 06:21 PM
  5. Help Isolating a Variable
    Posted in the Calculus Forum
    Replies: 1
    Last Post: November 2nd 2009, 03:10 PM

Search Tags


/mathhelpforum @mathhelpforum