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Formula for surface area with respect to x or y. Why is the radius for both y?



The Next CEO of Stack OverflowWhy can't I use the disk method to compute surface area?Integrating the formula for a circle's areaWhy do we care about the 'rapidness' for convergence?Volume by rotation: Cyndrilical shells and radiiArea below x axis not considered when finding volume of shape rotated around y axis?Integration of x^(1/2) sinxWhy is surface area not simply $2 pi int_a^b (y) dx$ instead of $2 pi int_a^b (y cdot sqrt1 + y'^2) dx$?Should the Volume of a Solid of Revolution always be the same when rotated around the x-axis than around y-axis?Volume of solid revolution 2Why does wolfram answer as such in this example for surface area and volume of revolution on an area crossing the axis?










0












$begingroup$


I am reading this text in the stewart text:



enter image description here



and I'm wondering if the text here is correct. For (6), why is it $2pi*y$ and not $2 pi x$? Isn't x rotated around the y axis since it's a function of y? I guess that's not true since just because a function is a function of y doesn't mean x is the radius right? And we use $$fracdxdy$$ in 6 because that's what the function's derivative is since the function is written as a function of y right?










share|cite|improve this question









$endgroup$
















    0












    $begingroup$


    I am reading this text in the stewart text:



    enter image description here



    and I'm wondering if the text here is correct. For (6), why is it $2pi*y$ and not $2 pi x$? Isn't x rotated around the y axis since it's a function of y? I guess that's not true since just because a function is a function of y doesn't mean x is the radius right? And we use $$fracdxdy$$ in 6 because that's what the function's derivative is since the function is written as a function of y right?










    share|cite|improve this question









    $endgroup$














      0












      0








      0





      $begingroup$


      I am reading this text in the stewart text:



      enter image description here



      and I'm wondering if the text here is correct. For (6), why is it $2pi*y$ and not $2 pi x$? Isn't x rotated around the y axis since it's a function of y? I guess that's not true since just because a function is a function of y doesn't mean x is the radius right? And we use $$fracdxdy$$ in 6 because that's what the function's derivative is since the function is written as a function of y right?










      share|cite|improve this question









      $endgroup$




      I am reading this text in the stewart text:



      enter image description here



      and I'm wondering if the text here is correct. For (6), why is it $2pi*y$ and not $2 pi x$? Isn't x rotated around the y axis since it's a function of y? I guess that's not true since just because a function is a function of y doesn't mean x is the radius right? And we use $$fracdxdy$$ in 6 because that's what the function's derivative is since the function is written as a function of y right?







      integration






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Mar 18 at 20:17









      Jwan622Jwan622

      2,32211632




      2,32211632




















          1 Answer
          1






          active

          oldest

          votes


















          1












          $begingroup$

          Yes, the text shown has a typo in equation 6; it should read $2pi x$ rather than $2pi y$.






          share|cite|improve this answer











          $endgroup$












          • $begingroup$
            do others agree??? Or am I wrong for thinking this?
            $endgroup$
            – Jwan622
            Mar 18 at 20:34










          • $begingroup$
            I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
            $endgroup$
            – Jwan622
            Mar 22 at 19:00











          Your Answer





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          1 Answer
          1






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          oldest

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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          Yes, the text shown has a typo in equation 6; it should read $2pi x$ rather than $2pi y$.






          share|cite|improve this answer











          $endgroup$












          • $begingroup$
            do others agree??? Or am I wrong for thinking this?
            $endgroup$
            – Jwan622
            Mar 18 at 20:34










          • $begingroup$
            I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
            $endgroup$
            – Jwan622
            Mar 22 at 19:00















          1












          $begingroup$

          Yes, the text shown has a typo in equation 6; it should read $2pi x$ rather than $2pi y$.






          share|cite|improve this answer











          $endgroup$












          • $begingroup$
            do others agree??? Or am I wrong for thinking this?
            $endgroup$
            – Jwan622
            Mar 18 at 20:34










          • $begingroup$
            I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
            $endgroup$
            – Jwan622
            Mar 22 at 19:00













          1












          1








          1





          $begingroup$

          Yes, the text shown has a typo in equation 6; it should read $2pi x$ rather than $2pi y$.






          share|cite|improve this answer











          $endgroup$



          Yes, the text shown has a typo in equation 6; it should read $2pi x$ rather than $2pi y$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Mar 18 at 21:17









          Robert Howard

          2,2783935




          2,2783935










          answered Mar 18 at 20:29









          Mark FischlerMark Fischler

          33.8k12552




          33.8k12552











          • $begingroup$
            do others agree??? Or am I wrong for thinking this?
            $endgroup$
            – Jwan622
            Mar 18 at 20:34










          • $begingroup$
            I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
            $endgroup$
            – Jwan622
            Mar 22 at 19:00
















          • $begingroup$
            do others agree??? Or am I wrong for thinking this?
            $endgroup$
            – Jwan622
            Mar 18 at 20:34










          • $begingroup$
            I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
            $endgroup$
            – Jwan622
            Mar 22 at 19:00















          $begingroup$
          do others agree??? Or am I wrong for thinking this?
          $endgroup$
          – Jwan622
          Mar 18 at 20:34




          $begingroup$
          do others agree??? Or am I wrong for thinking this?
          $endgroup$
          – Jwan622
          Mar 18 at 20:34












          $begingroup$
          I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
          $endgroup$
          – Jwan622
          Mar 22 at 19:00




          $begingroup$
          I think I misunderstood something about what limits of integration to use... if the arc length formula that I use uses dy, then the limits of integration have to be for y. Conversely, if the arc length formula I use uses dx, then the limits have to be for x right?
          $endgroup$
          – Jwan622
          Mar 22 at 19:00

















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