Parametric equation of an intersection.

Have a great day!In summary, the parametric equations of the intersection line of two planes 2x - 3y - z + 1 = 0 and 3x - 2y + 3z - 4 = 0 are x = t, y = (9/11)t - 1/11, and z = (-5/11)t + 14/11. This can be checked by substituting these equations into the equations for the planes, resulting in 0 for all values of t.
  • #1
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Homework Statement


Find the parametric equations of the intersection line of two planes 2x - 3y - z + 1 = 0 and
3x - 2y + 3z - 4 = 0


Homework Equations


N/A


The Attempt at a Solution


First I'll label them:
2x - 3y - z + 1 = 0 [1]
3x - 2y + 3z - 4 = 0 [2]
Then I get rid of the z variable for now, and multiply [1] by 3 to do that, then eliminate by adding:
6x - 9y - 3z +3 = 0
3x - 2y + 3z - 4 = 0
_________________
9x - 11y - 1 = 0 [3]
Then I write y in terms of x:
y = (9/11)x - 1/11[4]
Then substitute [4] back into [1]:
2x - 3((9/11)x - 1/11) - z +1 = 0
2x - (27/11)x +3/11 - z + 1 = 0
Then write z in terms of x:
z = (-5/11)x + 14/11
Finally, I set x = t to write the parametric equations:
x = t
y = (9/11)t - 1/11
z = (-5/11)t + 14/11
However, this was the answer my book got:
x = (11/9)t + 1/9
y = t
z = (-5/11)t + 11/9

Can anyone help me figure out what I did wrong? I double checked all the tedious calculations, and they seem correct to me. Thanks in advance.
 
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  • #2
I believe you as well are correct, they just parameterized y and you did it with x.

Alternatively you could use 9x - 11y - 1 = 0 to get a point (x,y) and then find the cross-product of the perpendicular vectors of the first two planes to get the direction of the line.
 
  • #3
rock.freak667 said:
I believe you as well are correct, they just parameterized y and you did it with x.
I see. Thank you for the help.

rock.freak667 said:
Alternatively you could use 9x - 11y - 1 = 0 to get a point (x,y) and then find the cross-product of the perpendicular vectors of the first two planes to get the direction of the line.
Isn't the cross product only used for lines and planes in 3 dimensional space?
 
  • #4
Retribution said:
I see. Thank you for the help.


Isn't the cross product only used for lines and planes in 3 dimensional space?

A plane really only has meaning in 3d space ;) (esoteric examples aside)

With the tools you have, I think your approach is fine. When dealing with the gradient in Calc III some of this will come up again, but will be solved using a different approach.

The cross product of two vectors normal (perpindicular) to intersecting planes will result in a vector parallell to the line formed by intersection of the planes. Using this method, your answer checks out to be OK. If you don't get this concept, it's OK - your method works just as well.
 
  • #5
mege said:
A plane really only has meaning in 3d space ;) (esoteric examples aside)

With the tools you have, I think your approach is fine. When dealing with the gradient in Calc III some of this will come up again, but will be solved using a different approach.

The cross product of two vectors normal (perpindicular) to intersecting planes will result in a vector parallell to the line formed by intersection of the planes. Using this method, your answer checks out to be OK. If you don't get this concept, it's OK - your method works just as well.

Interesting. I do know that the normals of two planes are: (2, -3, -1) and (3, -2, 3), but how could I use their cross product to check if my answer is correct? This would be really helpful to do for an upcoming exam, as a question like this was worth quite a lot on a previous test of mine. Thanks in advance.
 
  • #6
The simplest way to check your answer is to put the parametric equations of the line back into the equations for the plane and see if they are satisfied for all t.

For example, you have
x = t
y = (9/11)t - 1/11
z = (-5/11)t + 14/11

and the equation of the first plane is 2x - 3y - z + 1 = 0.

Then 2t- 3((9/11)t- 1/11)- ((-5/11)t+ 14/11)+ 1= 2t- (27/11)t+ 3/11+ (5/11)t- 14//11+ 1= (2- 27/11+ 5/11)t+ 3/11- 14/11+ 1= (2- 22/11)t- 11/11+ 1= (2- 2)t- 1+ 1= 0 for all t.
 
  • #7
HallsofIvy said:
The simplest way to check your answer is to put the parametric equations of the line back into the equations for the plane and see if they are satisfied for all t.

For example, you have
x = t
y = (9/11)t - 1/11
z = (-5/11)t + 14/11

and the equation of the first plane is 2x - 3y - z + 1 = 0.

Then 2t- 3((9/11)t- 1/11)- ((-5/11)t+ 14/11)+ 1= 2t- (27/11)t+ 3/11+ (5/11)t- 14//11+ 1= (2- 27/11+ 5/11)t+ 3/11- 14/11+ 1= (2- 22/11)t- 11/11+ 1= (2- 2)t- 1+ 1= 0 for all t.

Thank you, I am extremely grateful. This will be very helpful for my upcoming exam.
 

FAQ: Parametric equation of an intersection.

1. What is a parametric equation of an intersection?

A parametric equation of an intersection is a mathematical representation of a point where two or more curves, lines, or surfaces intersect. It expresses the coordinates of the point in terms of one or more parameters, typically denoted by t or θ.

2. How is a parametric equation of an intersection different from a standard equation?

A standard equation represents a relationship between two variables, such as x and y, in terms of a single equation. A parametric equation, on the other hand, represents the relationship between two variables in terms of one or more parameters, allowing for a more flexible and dynamic representation of the intersection.

3. What are the advantages of using a parametric equation of an intersection?

Parametric equations allow for a more versatile and precise representation of an intersection. They can handle complex intersections that cannot be expressed by a single equation and can also handle changes in the intersection over time or with different conditions.

4. How do you solve a parametric equation of an intersection?

To solve a parametric equation of an intersection, you need to find the values of the parameters that satisfy both equations. This can be done by substituting the parameter values into each equation and solving for the corresponding variables. The resulting values will give the coordinates of the intersection point.

5. In what fields are parametric equations of intersections commonly used?

Parametric equations of intersections are commonly used in fields such as mathematics, physics, engineering, and computer graphics. They are particularly useful in applications involving curves, such as designing and analyzing the motion of objects or creating 3D models of complex surfaces.

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