Implicit differentiation questions












1












$begingroup$


I want to solve $dy/dx$ for the following:



$x^2 + y^2 = R^2$ where $R$ is a constant.



I know to use implicit differentiation, though I have a question. When I derive $R^2$, do I obtain $2R$ or 0?



Additionally, deriving $y^2$ with respect to x yields $2y (dy/dx)$? This is different from a partial derivative?



Thanks!










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$endgroup$












  • $begingroup$
    If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
    $endgroup$
    – K. Jiang
    Jan 9 at 19:47










  • $begingroup$
    You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
    $endgroup$
    – TheSimpliFire
    Jan 9 at 19:49


















1












$begingroup$


I want to solve $dy/dx$ for the following:



$x^2 + y^2 = R^2$ where $R$ is a constant.



I know to use implicit differentiation, though I have a question. When I derive $R^2$, do I obtain $2R$ or 0?



Additionally, deriving $y^2$ with respect to x yields $2y (dy/dx)$? This is different from a partial derivative?



Thanks!










share|cite|improve this question









$endgroup$












  • $begingroup$
    If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
    $endgroup$
    – K. Jiang
    Jan 9 at 19:47










  • $begingroup$
    You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
    $endgroup$
    – TheSimpliFire
    Jan 9 at 19:49
















1












1








1





$begingroup$


I want to solve $dy/dx$ for the following:



$x^2 + y^2 = R^2$ where $R$ is a constant.



I know to use implicit differentiation, though I have a question. When I derive $R^2$, do I obtain $2R$ or 0?



Additionally, deriving $y^2$ with respect to x yields $2y (dy/dx)$? This is different from a partial derivative?



Thanks!










share|cite|improve this question









$endgroup$




I want to solve $dy/dx$ for the following:



$x^2 + y^2 = R^2$ where $R$ is a constant.



I know to use implicit differentiation, though I have a question. When I derive $R^2$, do I obtain $2R$ or 0?



Additionally, deriving $y^2$ with respect to x yields $2y (dy/dx)$? This is different from a partial derivative?



Thanks!







derivatives






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share|cite|improve this question










asked Jan 9 at 19:45









appleapple

445




445












  • $begingroup$
    If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
    $endgroup$
    – K. Jiang
    Jan 9 at 19:47










  • $begingroup$
    You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
    $endgroup$
    – TheSimpliFire
    Jan 9 at 19:49




















  • $begingroup$
    If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
    $endgroup$
    – K. Jiang
    Jan 9 at 19:47










  • $begingroup$
    You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
    $endgroup$
    – TheSimpliFire
    Jan 9 at 19:49


















$begingroup$
If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
$endgroup$
– K. Jiang
Jan 9 at 19:47




$begingroup$
If you are looking for $frac{dy}{dx},$ then you need to derive the given equation in terms of $x.$ $R$ is a constant. Notice that $frac{d}{dx} c = 0.$
$endgroup$
– K. Jiang
Jan 9 at 19:47












$begingroup$
You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
$endgroup$
– TheSimpliFire
Jan 9 at 19:49






$begingroup$
You are differentiating with respect to $x$, and since $R$ is independent of $x$, its derivative is zero. For the derivative of $y^2=f(x)^2$, use the chain rule; its derivative is $dfrac{d(y^2)}{df(x)}cdotdfrac{df(x)}x$
$endgroup$
– TheSimpliFire
Jan 9 at 19:49












2 Answers
2






active

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1












$begingroup$

By the chaine rule you will get $$2x+2ycdot y'=0$$






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    Suppose $R$ is a function of $x$ and $y$; then



    $x^2 + y^2 = R^2(x, y); tag 1$



    if we define



    $F(x, y) = x^2 + y^2 - R^2(x, y), tag 2$



    we may also write (1) as



    $F(x, y) = x^2 + y^2 - R^2(x, y) = 0; tag 3$



    by the implicit funtion theorem, this equation in fact may be seen as defining $y(x)$, a function of $x$, provided that



    $dfrac{partial F(x, y)}{partial y} ne 0; tag 4$



    we have



    $dfrac{partial F(x, y)}{partial y} = 2y - 2R(x, y) dfrac{partial R(x, y)}{partial y} ne 0 tag 5$



    provided



    $y ne R(x, y) dfrac{partial R(x, y)}{partial y};tag 6$



    under such circumstances, we may affirm $y(x)$ is uniquely determined as a differentiable function of $x$ in some neighborhood of any point $(x, y)$; then we have



    $F(x, y) = x^2 + y^2(x) - R^2(x, y(x)) = 0; tag 7$



    we may take the total derivative with respect to $x$ to obtain



    $dfrac{dF(x, y)}{dx} = 2x + 2ydfrac{dy(x)}{dx} - 2R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 8$



    a little algebra allows us to isolate the terms containing $dy(x)/dx$:



    $x + ydfrac{dy(x)}{dx} - R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 9$



    $ydfrac{dy(x)}{dx} - R(x, y(x)) dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{10}$



    $left ( y - R(x, y(x)) dfrac{partial R(x, y(x)}{partial y} right )dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{11}$



    for the sake of compactess and brevity, we introduce the subscript notation



    $R_x = dfrac{partial R}{partial x}, ; text{etc.}, tag{12}$



    and write (11) in the form



    $y'(x) = dfrac{RR_x - x}{y - RR_y} = -dfrac{x - RR_x}{y - RR_y}, tag{13}$



    which gives a general expression for $y'(x)$; in the event that $R(x, y)$ is constant, we obtain



    $y'(x) = -dfrac{x}{y}, tag{14}$



    which the reader may recognize as the slope of the circle



    $x^2 + y^2 = R^2 tag{15}$



    at any point $(x, y)$ where $y ne 0$.






    share|cite|improve this answer









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      2 Answers
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      2 Answers
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      1












      $begingroup$

      By the chaine rule you will get $$2x+2ycdot y'=0$$






      share|cite|improve this answer









      $endgroup$


















        1












        $begingroup$

        By the chaine rule you will get $$2x+2ycdot y'=0$$






        share|cite|improve this answer









        $endgroup$
















          1












          1








          1





          $begingroup$

          By the chaine rule you will get $$2x+2ycdot y'=0$$






          share|cite|improve this answer









          $endgroup$



          By the chaine rule you will get $$2x+2ycdot y'=0$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 9 at 19:47









          Dr. Sonnhard GraubnerDr. Sonnhard Graubner

          74.9k42865




          74.9k42865























              0












              $begingroup$

              Suppose $R$ is a function of $x$ and $y$; then



              $x^2 + y^2 = R^2(x, y); tag 1$



              if we define



              $F(x, y) = x^2 + y^2 - R^2(x, y), tag 2$



              we may also write (1) as



              $F(x, y) = x^2 + y^2 - R^2(x, y) = 0; tag 3$



              by the implicit funtion theorem, this equation in fact may be seen as defining $y(x)$, a function of $x$, provided that



              $dfrac{partial F(x, y)}{partial y} ne 0; tag 4$



              we have



              $dfrac{partial F(x, y)}{partial y} = 2y - 2R(x, y) dfrac{partial R(x, y)}{partial y} ne 0 tag 5$



              provided



              $y ne R(x, y) dfrac{partial R(x, y)}{partial y};tag 6$



              under such circumstances, we may affirm $y(x)$ is uniquely determined as a differentiable function of $x$ in some neighborhood of any point $(x, y)$; then we have



              $F(x, y) = x^2 + y^2(x) - R^2(x, y(x)) = 0; tag 7$



              we may take the total derivative with respect to $x$ to obtain



              $dfrac{dF(x, y)}{dx} = 2x + 2ydfrac{dy(x)}{dx} - 2R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 8$



              a little algebra allows us to isolate the terms containing $dy(x)/dx$:



              $x + ydfrac{dy(x)}{dx} - R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 9$



              $ydfrac{dy(x)}{dx} - R(x, y(x)) dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{10}$



              $left ( y - R(x, y(x)) dfrac{partial R(x, y(x)}{partial y} right )dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{11}$



              for the sake of compactess and brevity, we introduce the subscript notation



              $R_x = dfrac{partial R}{partial x}, ; text{etc.}, tag{12}$



              and write (11) in the form



              $y'(x) = dfrac{RR_x - x}{y - RR_y} = -dfrac{x - RR_x}{y - RR_y}, tag{13}$



              which gives a general expression for $y'(x)$; in the event that $R(x, y)$ is constant, we obtain



              $y'(x) = -dfrac{x}{y}, tag{14}$



              which the reader may recognize as the slope of the circle



              $x^2 + y^2 = R^2 tag{15}$



              at any point $(x, y)$ where $y ne 0$.






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                Suppose $R$ is a function of $x$ and $y$; then



                $x^2 + y^2 = R^2(x, y); tag 1$



                if we define



                $F(x, y) = x^2 + y^2 - R^2(x, y), tag 2$



                we may also write (1) as



                $F(x, y) = x^2 + y^2 - R^2(x, y) = 0; tag 3$



                by the implicit funtion theorem, this equation in fact may be seen as defining $y(x)$, a function of $x$, provided that



                $dfrac{partial F(x, y)}{partial y} ne 0; tag 4$



                we have



                $dfrac{partial F(x, y)}{partial y} = 2y - 2R(x, y) dfrac{partial R(x, y)}{partial y} ne 0 tag 5$



                provided



                $y ne R(x, y) dfrac{partial R(x, y)}{partial y};tag 6$



                under such circumstances, we may affirm $y(x)$ is uniquely determined as a differentiable function of $x$ in some neighborhood of any point $(x, y)$; then we have



                $F(x, y) = x^2 + y^2(x) - R^2(x, y(x)) = 0; tag 7$



                we may take the total derivative with respect to $x$ to obtain



                $dfrac{dF(x, y)}{dx} = 2x + 2ydfrac{dy(x)}{dx} - 2R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 8$



                a little algebra allows us to isolate the terms containing $dy(x)/dx$:



                $x + ydfrac{dy(x)}{dx} - R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 9$



                $ydfrac{dy(x)}{dx} - R(x, y(x)) dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{10}$



                $left ( y - R(x, y(x)) dfrac{partial R(x, y(x)}{partial y} right )dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{11}$



                for the sake of compactess and brevity, we introduce the subscript notation



                $R_x = dfrac{partial R}{partial x}, ; text{etc.}, tag{12}$



                and write (11) in the form



                $y'(x) = dfrac{RR_x - x}{y - RR_y} = -dfrac{x - RR_x}{y - RR_y}, tag{13}$



                which gives a general expression for $y'(x)$; in the event that $R(x, y)$ is constant, we obtain



                $y'(x) = -dfrac{x}{y}, tag{14}$



                which the reader may recognize as the slope of the circle



                $x^2 + y^2 = R^2 tag{15}$



                at any point $(x, y)$ where $y ne 0$.






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  Suppose $R$ is a function of $x$ and $y$; then



                  $x^2 + y^2 = R^2(x, y); tag 1$



                  if we define



                  $F(x, y) = x^2 + y^2 - R^2(x, y), tag 2$



                  we may also write (1) as



                  $F(x, y) = x^2 + y^2 - R^2(x, y) = 0; tag 3$



                  by the implicit funtion theorem, this equation in fact may be seen as defining $y(x)$, a function of $x$, provided that



                  $dfrac{partial F(x, y)}{partial y} ne 0; tag 4$



                  we have



                  $dfrac{partial F(x, y)}{partial y} = 2y - 2R(x, y) dfrac{partial R(x, y)}{partial y} ne 0 tag 5$



                  provided



                  $y ne R(x, y) dfrac{partial R(x, y)}{partial y};tag 6$



                  under such circumstances, we may affirm $y(x)$ is uniquely determined as a differentiable function of $x$ in some neighborhood of any point $(x, y)$; then we have



                  $F(x, y) = x^2 + y^2(x) - R^2(x, y(x)) = 0; tag 7$



                  we may take the total derivative with respect to $x$ to obtain



                  $dfrac{dF(x, y)}{dx} = 2x + 2ydfrac{dy(x)}{dx} - 2R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 8$



                  a little algebra allows us to isolate the terms containing $dy(x)/dx$:



                  $x + ydfrac{dy(x)}{dx} - R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 9$



                  $ydfrac{dy(x)}{dx} - R(x, y(x)) dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{10}$



                  $left ( y - R(x, y(x)) dfrac{partial R(x, y(x)}{partial y} right )dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{11}$



                  for the sake of compactess and brevity, we introduce the subscript notation



                  $R_x = dfrac{partial R}{partial x}, ; text{etc.}, tag{12}$



                  and write (11) in the form



                  $y'(x) = dfrac{RR_x - x}{y - RR_y} = -dfrac{x - RR_x}{y - RR_y}, tag{13}$



                  which gives a general expression for $y'(x)$; in the event that $R(x, y)$ is constant, we obtain



                  $y'(x) = -dfrac{x}{y}, tag{14}$



                  which the reader may recognize as the slope of the circle



                  $x^2 + y^2 = R^2 tag{15}$



                  at any point $(x, y)$ where $y ne 0$.






                  share|cite|improve this answer









                  $endgroup$



                  Suppose $R$ is a function of $x$ and $y$; then



                  $x^2 + y^2 = R^2(x, y); tag 1$



                  if we define



                  $F(x, y) = x^2 + y^2 - R^2(x, y), tag 2$



                  we may also write (1) as



                  $F(x, y) = x^2 + y^2 - R^2(x, y) = 0; tag 3$



                  by the implicit funtion theorem, this equation in fact may be seen as defining $y(x)$, a function of $x$, provided that



                  $dfrac{partial F(x, y)}{partial y} ne 0; tag 4$



                  we have



                  $dfrac{partial F(x, y)}{partial y} = 2y - 2R(x, y) dfrac{partial R(x, y)}{partial y} ne 0 tag 5$



                  provided



                  $y ne R(x, y) dfrac{partial R(x, y)}{partial y};tag 6$



                  under such circumstances, we may affirm $y(x)$ is uniquely determined as a differentiable function of $x$ in some neighborhood of any point $(x, y)$; then we have



                  $F(x, y) = x^2 + y^2(x) - R^2(x, y(x)) = 0; tag 7$



                  we may take the total derivative with respect to $x$ to obtain



                  $dfrac{dF(x, y)}{dx} = 2x + 2ydfrac{dy(x)}{dx} - 2R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 8$



                  a little algebra allows us to isolate the terms containing $dy(x)/dx$:



                  $x + ydfrac{dy(x)}{dx} - R(x, y(x)) left ( dfrac{partial R(x, y(x))}{partial x} + dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} right ) = 0; tag 9$



                  $ydfrac{dy(x)}{dx} - R(x, y(x)) dfrac{partial R(x, y(x))}{partial y} dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{10}$



                  $left ( y - R(x, y(x)) dfrac{partial R(x, y(x)}{partial y} right )dfrac{dy(x)}{dx} = R(x, y(x))dfrac{partial R(x, y(x))}{partial x} - x; tag{11}$



                  for the sake of compactess and brevity, we introduce the subscript notation



                  $R_x = dfrac{partial R}{partial x}, ; text{etc.}, tag{12}$



                  and write (11) in the form



                  $y'(x) = dfrac{RR_x - x}{y - RR_y} = -dfrac{x - RR_x}{y - RR_y}, tag{13}$



                  which gives a general expression for $y'(x)$; in the event that $R(x, y)$ is constant, we obtain



                  $y'(x) = -dfrac{x}{y}, tag{14}$



                  which the reader may recognize as the slope of the circle



                  $x^2 + y^2 = R^2 tag{15}$



                  at any point $(x, y)$ where $y ne 0$.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Jan 9 at 23:34









                  Robert LewisRobert Lewis

                  45.7k23065




                  45.7k23065






























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