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Math Help - Showing that the tangent to the curve is also tangent to curve at another point.

  1. #1
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    Showing that the tangent to the curve is also tangent to curve at another point.

    Show that the tangent to the curve y=(x^2 +x -2)^2 +3 at the point where x=1 is also tangent to the curve at another point.

    So far this is what I did:

    Subbed in 1 to the oringinal curve to find out that the point is (1,3)

    Than I derived it to 4x^3 +6x^2 -6x -4 and when I subbed in 1 into the derivative I got 0.

    Than I found out the equation of the line which is y=3, but i'm completely lost on what to do from there
    Last edited by kmjt; April 5th 2010 at 07:14 PM.
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  2. #2
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    Quote Originally Posted by kmjt View Post
    Show that the tangent to the curve y=(x^2 +x -2)^2 +3 at the point where x=1 is also tangent to the curve at another point.

    So far this is what I did:

    Subbed in 1 to the oringinal curve to find out that the point is (1,3)

    Than I derived it to 4x^2 +6x^2 -6x -4 and when I subbed in 1 into the derivative I got 0.

    Than I found out the equation of the line which is y=3, but i'm completely lost on what to do from there
    Yes, that is correct except for a small typo.

    f'(x)=2\left(x^2+x-2\right)(2x+1)=4x^3+6x^2-6x-4

    At x=1, f'(x)=0.

    x=1, y=3. The tangent at x=1 is parallel to the x-axis
    (it's a local minimum).

    If this tangent is tangent at another point, then there is a 2nd turning point at y=3.

    Hence, we find a second solution for x, if y=3 corresponds to f'(x)=0 for more than one x.

    This also means we can examine f(x) to see how many x causes f(x)=3.

    f(x)=\left(x^2+x-2\right)^2+3=3\ \Rightarrow\ \left(x^2+x-2\right)=0

    \Rightarrow\ (x+2)(x-1)=0

    f(x)=3 for x=1 and x=-2 only.

    To check the tangent, if x=-2, f'(x)=-32+16+16+8-4-4=0.

    Hence, The tangent is also tangent to the curve at (-2,3).
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  3. #3
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    Hmm i'm kind of confused at this point. How are you going from (x^2 +x -2)^2 +3 to (x^2 +x -2)?
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    f(x)=(x^2 +x-2)^2 + 3 = 3 \Rightarrow (x^2+x-2)^2 = 3-3 \Rightarrow (x^2 + x-2)^2 = 0

    (x^2 + x-2)^2 = (x+2)^2 (x-1)^2
    Also
    (x^2 + x-2) = (x+2) (x-1)

    They have the same roots, therefore it will yield the same answer, so this is why he is jumping from:
    (x^2 + x-2)^2 + 3 = 3 \Rightarrow (x^2 + x-2) = 0

    Hope this helps.
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  5. #5
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    Quote Originally Posted by kmjt View Post


    Hmm i'm kind of confused at this point. How are you going from (x^2 +x -2)^2 +3 to (x^2 +x -2)?
    Yes,

    f(x)=\left(x^2+x-2\right)^2+3=3

    means

    \left(x^2+x-2\right)^2=0\ \Rightarrow\ \left(x^2+x-2\right)\left(x^2+x-2\right)=0

    \Rightarrow\ \left(x^2+x-2\right)=0\ \Rightarrow\ (x+2)(x-1)=0

    x=1

    x=-2
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  6. #6
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    Why wouldn't you expand (x^2 +x -2)(x^2 +x -2)?
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  7. #7
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    Quote Originally Posted by kmjt View Post
    Why wouldn't you expand (x^2 +x -2)(x^2 +x -2)?
    Hi kmjt,

    we could, but there is no need to!
    it's just extra work...

    f(x)=\left(x^2+x-2\right)\left(x^2+x-2\right)+3

    If f(x)=3, then f(x)=0+3, the product of the brackets must be zero,
    hence even if we multiply out the brackets, all that will still be zero
    at the second point on the graph that that particular tangent touches.

    Since it is the same factor multiplied by itself,
    we are left to solve

    x^2+x-2=0.

    since 0(0)=0

    As this is a quadratic, it gives us at most 2 values of x causing f(x)=3.
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