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

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Discussion Overview

The discussion revolves around whether lines or curves in three-dimensional space must be represented by parametric equations. Participants explore the nature of curves, the relationship between equations and geometric representations, and the implications of using different forms of equations.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that in three-dimensional space, equations using x, y, and z will define a plane or surface, and thus lines or curves must be represented parametrically as x(t), y(t), and z(t).
  • Another participant presents a set of equations (x² + y² = 1, z = 2) as defining a curve in R³, implying that non-parametric forms can represent curves.
  • A participant mentions that calculus textbooks typically represent lines and curves in three-dimensional space using parametric equations, suggesting a common pedagogical approach.
  • Some participants argue that curves do not necessarily have to be represented by parametric equations, indicating alternative representations are possible.
  • One participant explains that any time there are n equations in n+1 variables, it is theoretically possible to express n variables in terms of one, which can lead to parametric forms.
  • Another participant discusses the "symmetric form" of equations as an alternative way to represent curves, emphasizing that two equations in three variables can yield a one-dimensional representation.

Areas of Agreement / Disagreement

Participants express differing views on whether parametric equations are necessary for representing curves in three-dimensional space. Some argue for the necessity of parametric forms, while others contend that alternative representations exist. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference various mathematical forms and representations, but there is no consensus on the necessity of parametric equations for curves in three-dimensional space. The discussion highlights the theoretical flexibility in representing curves and the implications of different mathematical approaches.

yungman
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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?
 
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The set of solutions of these two equation defines a curve in R3:
x2 + y2 = 1
z = 2
 
Mark44 said:
The set of solutions of these two equation defines a curve in R3:
x2 + y2 = 1
z = 2

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.
 
My point is that curves in space don't have to be represented by parametric equations.
 
Thanks.
Alan
 
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.
 
HallsofIvy said:
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.

Thanks a million, this is exactly what I need, a comparison.

Alan
 

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