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Is equation of line or curve in 3 space have to be a parametric equation?

  1. Aug 3, 2010 #1
    I have been confused about this. When come to equation of lines and curves in even 2 space, they automatically go parametric equation x(t), y(t) etc. I just want to verify my understanding:

    For 3 space, equation using x, y and z will automatically produce a plane or a surface. In order to produce a line or curve, they have not choice but to represent x, y and z in form of x(t), y(t) and z(t) so it become line or curve.

    Therefore a linear line or a curve HAS to be represented by a parametric equation.

    Am I correct?
  2. jcsd
  3. Aug 3, 2010 #2


    Staff: Mentor

    The set of solutions of these two equation defines a curve in R3:
    x2 + y2 = 1
    z = 2
  4. Aug 3, 2010 #3
    In the calculus books I studied include Sherman Stein and Howard Anton. They only represent lines and curves in 3 space with parametric equation. Also in the chapter of vector value function. All the books I have immediately go to parametric equations.
  5. Aug 3, 2010 #4


    Staff: Mentor

    My point is that curves in space don't have to be represented by parametric equations.
  6. Aug 3, 2010 #5
  7. Aug 4, 2010 #6


    User Avatar
    Science Advisor

    Curves are one dimensional. That means that every point can, theoretically, be determined by using a single number. If you write a curve in terms of parametric equations, then that single number, the parameter, is apparent.

    But any time you have n equations in n+1 variables, you can, theoretically, solve for n of the variables in terms of the other one. For example, if you have 3x+ 2y+ z= 4 and x- y- z= 5, adding the two equations eliminates z giving 4x+ y= 9. You can then solve for y in terms of x: y= 9- 4x. Putting that into the second equation, x- (9- 4x)- z= -3x- 9- z= 5 or -z= 14+ 3x so that z= -14- 3x. We could then use x as parameter:
    x= t, y= 9- 4t, z= -14- 3t. But the two equations 3x+ 2y+ z= 4 and x- y- z= 5 represent that line (geometrically you can think of the line as the intersection of the two planes given by those equations.

    Another way of representing a curve is in what is called the "symmetric form": f(x,y,z)= g(x, y, z)= h(x, y, z). That is just a way of representing the two equations f(x,y,z)= g(x,y,z), g(x,y,z)= h(x,y,z). Again, since there are two equations in three variables, you could, theoretically, solve for two of the variables in terms of the third- its graph is one-dimensional, a curve.
  8. Aug 4, 2010 #7
    Thanks a million, this is exactly what I need, a comparison.

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