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Thread: Help with a problem.

  1. #1
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    Help with a problem.

    How do I find all points on a curve whose x coordinate is 1? Then, how do I write an equation for the tangent line at each of these points?

    The curve is xy^2-x^3y=6

    & I already have the derivative.
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  2. #2
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by MadHotThunder View Post
    How do I find all points on a curve whose x coordinate is 1? Then, how do I write an equation for the tangent line at each of these points?

    The curve is xy^2-x^3y=6

    & I already have the derivative.
    To answer the first question:

    Plug in $\displaystyle x=1$ into the original equation to find the y values.

    $\displaystyle y^2-y=6\implies y^2-y-6=0$

    This can be easily factored. You will get two y values, which I will denote as $\displaystyle y_1$ and $\displaystyle y_2$ respectively.

    This will give you the two points: $\displaystyle (1,y_1)$ and $\displaystyle (1,y_2)$

    Now, find the derivative [which you already did] and then plug these points into $\displaystyle \frac{\,dy}{\,dx}$. Each point will create a different slope, $\displaystyle m_1$ and $\displaystyle m_2$, respectively.

    Now to find the equations of the tangents, use the point slope form of a line. You will have two equations:

    $\displaystyle y-y_1=m_1(x-1)$ and $\displaystyle y-y_2=m_2(x-1)$

    Can you take it from here?

    --Chris
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  3. #3
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    Quote Originally Posted by Chris L T521 View Post
    To answer the first question:

    Plug in $\displaystyle x=1$ into the original equation to find the y values.

    $\displaystyle y^2-y=6\implies y^2-y-6=0$

    This can be easily factored. You will get two y values, which I will denote as $\displaystyle y_1$ and $\displaystyle y_2$ respectively.

    This will give you the two points: $\displaystyle (1,y_1)$ and $\displaystyle (1,y_2)$

    Now, find the derivative [which you already did] and then plug these points into $\displaystyle \frac{\,dy}{\,dx}$. Each point will create a different slope, $\displaystyle m_1$ and $\displaystyle m_2$, respectively.

    Now to find the equations of the tangents, use the point slope form of a line. You will have two equations:

    $\displaystyle y-y_1=m_1(x-1)$ and $\displaystyle y-y_2=m_2(x-1)$

    Can you take it from here?

    --Chris

    You are AMAZING.
    One last thing, how would I find the x-coordinate of each point on the cure where the tangent line is vertical??
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