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Homework Help: Intersection of planes, curvature and osculating plane

  1. Mar 5, 2012 #1
    1. The problem statement, all variables and given/known data
    The equations sin(xyz) = 0 and x + xy + z^3 = 0 define planes in R^3. Find the osculating plane and the curvature of the intersection of the curves at (1, 0, -1)

    2. Relevant equations
    Osculating plane of a curve = {f + s*f' + t*f'' : s, r are reals}
    Curvature = ||T'|| where T is the unit tangent vector

    3. The attempt at a solution
    I guess my biggest doubt here is determining the position vector I want to be working with. Since we're looking at the point (1, 0, -1), then sin(zyx) = 0 implies that y=0. (I got this hint but I don't really understand it). So now we got the intersection x + z^3 = 0, and if we parametrize x(t) = t^3, z(t) = -t and y(t) = 0 then we get r(t) = t^3 * i - t * k where (i, j, k) is the standard basis for R^3. Now we differentiate and take lengths in turns of r to get the vectors we want to work with to get the curvature and osculating plane. Is this the correct method?
  2. jcsd
  3. Mar 5, 2012 #2


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    Science Advisor

    I assume you mean "intersection of the planes at (1, 0, -1)"

    That was given as a "hint"? It isn't true. At (1, 0, -1), yes y= 0 at that point. It does not follow that y= 0 at any other point on the curve of intersection.
    From x+ xy+ z^3= 0, [itex]z= -x^{1/3}(1+ y)^{1/3}[/itex] so the second equation , sin(xyz)= 0, becomes [itex]sin(x^{4/3}y(1+ y)^{1/3})= 0[/itex]. Differentiate with respect to x and y.

  4. Mar 5, 2012 #3
    The hint wasn't from an instructor but an older student, seems it was incorrect.
    If we differentiate with respect to x we get

    d/(dx) sin(x^4/(3 y) (1+y)×1/3) = (4 x^3 (y+1) cos((x^4 (y+1))/(9 y)))/(9 y) = 0.

    and if we differentiate with respect to y we get

    d/(dy) sin(x^4/(3 y) (1+y)×1/3) = -(x^4 cos((x^4 (y+1))/(9 y)))/(9 y^2) = 0

    I've only looked at examples where we work with position vectors of the form r(t) = x(t) * i + y(t) * j + z(t) * k, so I don't know what to do with the partial derivatives!
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