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Homework Help: Tangent Lines to an Ellipse Passing Through an Outside Point

  1. Feb 15, 2012 #1
    1. The problem statement, all variables and given/known data
    Find equations of both the tangent lines to the ellipse
    x2 + 4y2 = 36
    that pass through the point (12, 3).

    2. Relevant equations
    The equation of an ellipse is x2/a2 + y2/b2 = 1.
    I converted the given equation to
    x2/36 + y2/9 = 1 by dividing each value by 36.

    3. The attempt at a solution
    My online homework website is telling me I'm half right (I found the equation of ONE of the tangent lines to the ellipse passing through (12,3)). Here's what I have:

    I took the derivative of the equation of the ellipse and got y' = -x/4y.
    I set this value equal to the slope of a line passing through the point (12,3), (y-3)/(x-12). I then solved for x2 and y2 to find

    x2=-4y2 + 12y + 12x
    y2=-x2/4 + 3x + 3y

    I then substituted these equations, one at a time, for the values of x2 and y2 in the original equation of the ellipse and solved for the unknown value. I think I took an unnecessary step doing that, but I need the practice! I found that


    I then determined the slopes of the lines passing through (0,3)-->(12,3) and (0,-3)-->(12,3). Clearly, the line running from (0,3)-->(12,3) has a slope of 0, and the equation of the line is y=3. This is the part that I got correct.
    The slope of the line running through (0,-3)-->(12,3) I found to be 1/2. However, my homework website is telling me the equation y=1/2x-3 is incorrect.

    Any assistance you can give me would be very appreciated! Honestly, we haven't gone over ANY problems like this in class, so the fact I'm even half right has been enormously satisfying. But I'd like to know what I'm doing wrong... I have a feeling I probably did something stupid, like dropped a negative sign somewhere. There are only 6 questions on this section's homework, so as it is I have an 83.3% and can't make an A unless I get this last piece of the last question correct. So frustrating!

    Thanks in advance!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 15, 2012 #2


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    Homework Helper

    If you got x^2=-4y^2 + 12y + 12x from your slope relation, then that tells you x^2+4y^2=12y+12x. x^2+4y^2=36. So 36=12y+12x. Solve that for y and substitute back into the ellipse equation. You should get two values of x that satisfy it. Not just x=0.
    Last edited: Feb 15, 2012
  4. Feb 15, 2012 #3
    Thanks! Of course it was right in front of me!
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