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Homework Help: Find two lines through the point (2,8) and tangent to y = x^3

  1. Oct 26, 2006 #1
    Find two lines that pass through the point[tex](2,8)[/tex] and are tangent to the curve [tex]y=x^3[/tex]

    I found the first equation which is [tex]y=12x-16[/tex] by finding the derivative of [tex]y=x^3[/tex] which is [tex]y=3x^2[/tex].

    I then plugged in [tex](2,8)[/tex] into [tex]y=mx+b[/tex] along with the slope of [tex]12[/tex].

    But how can there be more than one slope at a single point? I don't get it
  2. jcsd
  3. Oct 27, 2006 #2


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    Does the text say that the lines has to be tangent to the given curve AT the point (2,8)?
    From how I read it, the text just requires that the lines go THROUGH (2,8)
  4. Oct 27, 2006 #3
    isn't that the same thing....

    and there only should be 1 tangent for an equation where the answer theres 2
  5. Oct 28, 2006 #4
    arildno is saying that the problem not not be asking for two tangent lines through the same point, but instead a tangential line and a non-tangential line to (2,8). If that's the case, this is simple to do. You already have the tangent's equation, so just make an equation for a different line that passes through (2,8) with a different slope.
  6. Oct 28, 2006 #5
    [tex]y - 8 = m(x-2) [/tex]
    [tex]y = (3x^2)(x-2) + 8 [/tex]
    [tex]x^3 = (3x^2)(x-2) + 8 [/tex]
    [tex]2x^3 - 6x^2 + 8 = 0 [/tex]

    Solve for x and then use this to find equation of the two lines.
    Last edited: Oct 28, 2006
  7. Oct 28, 2006 #6
    so both of them have to be tangent....yea i guess that makes sense what u said but it seems that is not what the question is asking
  8. Oct 28, 2006 #7


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    The second line has to be a tangent too, and it has to pass through the point (2, 8). So, these are two conditions.

    Use the equation of a line which passes through two points (x1, y1) and (x2, y2): y - y1 = ( (y2 - y1)/(x2 - x1) ) (x - x1). So, for example, set (x1, y1) = (2, 8). Further on, y2 must equal x2^3, and x2 is the point you need to find.

    Further on, the equation of a tangent line at a point c is:
    y - f(c) = f'(c) (x - c). So, set c = x2.

    Now bring both of the equations into the form y = m x + b. Set the two slopes equal, and you'll get your point x2, which you can plug back into any of the equations to obtain the other tangent.
  9. Oct 28, 2006 #8
    The method posted by alfredbester should work fine no? Solve the cubic, put the x-values back into equation for y. Then use (x2. y2) and (x3, y3) and y = mx + c.
    Last edited: Oct 28, 2006
  10. Oct 28, 2006 #9
    ok answers are: y=12x-16 and y=3x+2 that means that there are 2 slopes for the tangent at (2,8) which is impossible
  11. Oct 28, 2006 #10
    This isn't true at all. Draw the x^3 graph then try to draw straight lines that pass through a specific point, can you draw two lines that got through the point and then make a tangent to the curve? The line does not need to be tangent to the curve at the point (2,8) just pass the point (2,8) and later be tangent with the curve.
  12. Oct 30, 2006 #11
    so I got x=2 or a=2 but I'm stuck from here

    [tex]x^3=3x^2(2)+b\ \ \ \ \ \ \ \ or\ \ a^3=3a^2(2)+b\ \ \ \ \ \ \ \ \ 8=24+b, b= -16[/tex]

    [tex]y=12x-16[/tex] and I already got that...how do I get the other
    Last edited: Oct 30, 2006
  13. Oct 30, 2006 #12
    Stuck on what? You already have the equation of the two lines tangent to the curve that pass through (2,8).
  14. Oct 30, 2006 #13
    so I got x=2 or a=2 but I'm stuck from here

    [tex]x^3=3x^2(2)+b\ \ \ \ \ \ \ \ or\ \ a^3=3a^2(2)+b\ \ \ \ \ \ \ \ \ 8=24+b, b= -16[/tex]

    [tex]y=12x-16[/tex] and I already got that...how do I get the other
  15. Oct 30, 2006 #14

    So you have that, solve for x. You will get X= 2, -1.
    Then use y = mx + c, to find the two equations which you've already written down.
  16. Oct 30, 2006 #15
    holy christ I'm dumb, I only took the roots of (x-2)(x-2)

    thank you max
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