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Homework Help: Surface area of smooth parametric surface

  1. Feb 11, 2010 #1
    Sorry I am new to the forum, I don't know how to type in the integrals and stuff.

    1. The problem statement, all variables and given/known data
    Let S be the portion of the surface y = x2 where 0 <= z <= X <= 2. Compute the surface area of S.

    2. Relevant equations
    r(u,v) = x(u,v)i + y(u,v)j + z(u,v)k
    A(S) = integral ru X rv dA

    3. The attempt at a solution
    My first attempt I did: x = ucosv, y = u2cos2v, and z = ucosv where 0<= u <= 2 and 0 <= v <= 2pi
    when I was doing ru X rv, got a lot of sines and cosines but everything canceled out at the end and became 0.

    Then I just decided to use x = x, and y = x2 where x is from 0 to 2.
    but then ry turns out to be 0.

    I am not sure how I would do this, most of the problems I have done are z as a function of x and y, but this case z is given but I don't know what to do with it.

    Could some one please help me with this? Thank you for your help in advance.
  2. jcsd
  3. Feb 11, 2010 #2


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    Gold Member

    Unless you are describing a surface of revolution, which you haven't described, I don't see why you would involve trig functions in the parameterization. The surface y = x2 is a cylindrical parabola standing vertically on the xy plane. If I understand your description correctly, you are talking about that portion of the cylindrical surface in the first octant under the plane z = x.

    If I'm correct, the natural paramaterization would be to use x and z:

    [tex]\vec R(x,z) = \langle x,x^2,z\rangle[/tex]

    and your domain is a triangle in the xz plane. Try that.
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