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Finding all tangent lines through a point

  1. Feb 11, 2013 #1
    1. The problem statement, all variables and given/known data

    Find all tangent lines of the graph f(x)=x+3/x that have a y intercept of 4.

    2. Relevant equations



    3. The attempt at a solution

    Assume a is the x coordinate of a point of tangency. Thus the point of tangency is (a, a+3/a). We know the tangent line must pass through (0,4) so the slope of the line must be (a+3/a-4)/(a-0).

    f'(x)=1-3/(x^2)

    Derivative of f at point a must equal the slope of the tangent line, i.e. we must have
    f'(a)=1-3/(a^2)=(a+3/a-4)/(a-0)=m

    Solving I get a=3/2. However, looking at the graph of f(x), it seems there should be two points where the tangent line passes through 4, the other one being on the part of the f(x) where x<0. Where did I go wrong?
     
  2. jcsd
  3. Feb 11, 2013 #2

    Dick

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    I don't think you went wrong anywhere. Looking at the graph I don't see any points x<0 where the tangent line will go through (0,4).
     
  4. Feb 11, 2013 #3

    ehild

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    Gold Member

    If a is negative, the y intercepts of the tangent lines are also negative.

    ehild
     
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