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Math Help - Line intercepting curve

  1. #1
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    Line intercepting curve

    A straight line of gradient m is drawn through the point A(2, -4) on the curve y = x^2 - 4x. Find, in terms of m, the coordinates of the point B at which the line intercepts the curve again.
    These are the steps I have done thus far:
    Let c be the y - intercept
    -4 = 2m + c
    c = -4 - 2m
    Hence equation of the line is y = mx - 4 - 2m
    Equating this into y = x^2 - 4x,
    mx - 4 - 2m = x^2 - 4x
    x^2 - 4x - mx + 4 + 2m = 0
    x^2 - x(4 + m) + 4 + 2m = 0<br />
    I'm not sure how I can factorise this into the answer that is (2 + m, m^2 - 4)
    Any help would be greatly appreciated and thank you very much in advance!
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by caramelcake View Post
    These are the steps I have done thus far:
    Let c be the y - intercept
    -4 = 2m + c
    c = -4 - 2m
    Hence equation of the line is y = mx - 4 - 2m
    Equating this into y = x^2 - 4x,
    mx - 4 - 2m = x^2 - 4x
    x^2 - 4x - mx + 4 + 2m = 0
    x^2 - x(4 + m) + 4 + 2m = 0<br />
    I'm not sure how I can factorise this into the answer that is (2 + m, m^2 - 4)
    Any help would be greatly appreciated and thank you very much in advance!
    Use the quadratic formula to find the roots, when you do you will find that you do not have the given solution, so check your previous work (but it looks OK to me, so may be check that you are looking at the correct solution)

    CB
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  3. #3
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    Thank you CaptainBlack! But I have tried using the quadratic formula and can't seem to continue:
    (I got a latex error so forgive my handwriting)

    Does this mean that my answer should contain a surd?
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  4. #4
    Senior Member Educated's Avatar
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    How did you get (m^2 - 16m) inside the square root? Shouldn't it be just M^2?

    x = \dfrac{4+m\pm\sqrt{(-4-m)^2 - 4 * 1 * (4+2m)}}{2}

    This is what I got:

    x = \dfrac{4+m\pm\sqrt{16 + m^2 + 8m - 16-8m}}{2}

    x = \dfrac{4+m\pm\sqrt{m^2}}{2}


    x = \dfrac{4+m+m}{2} = 2+m

    x = \dfrac{4+m-m}{2} = 2

    Therefore the line will always hit the parabola at point 2, and also the point 2 + gradient.
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  5. #5
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    Thank you Educated! I have finished this sum now.
    @CaptainBlack
    Why isn't the textbook answer right then?
    Taking x to be 2 + m,
    y = m (2 + m) - 4 - 2m
    y = 2m + m^2 - 4 - 2m
    y = m^2 -4
    Hence the coordinates of point B are (2 + m, m^2 -4)
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  6. #6
    Grand Panjandrum
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    Quote Originally Posted by caramelcake View Post
    Thank you Educated! I have finished this sum now.
    @CaptainBlack
    Why isn't the textbook answer right then?
    Taking x to be 2 + m,
    y = m (2 + m) - 4 - 2m
    y = 2m + m^2 - 4 - 2m
    y = m^2 -4
    Hence the coordinates of point B are (2 + m, m^2 -4)
    My mistake, I misinterpreted what the text books answer was supposed to be.

    CB
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