# Finding line where two planes intersect

• ChiralSuperfields
In summary: In vector form, the above is ##L = \begin{bmatrix} \frac 3 7 \\ \frac 1 7 \\ 1 \end{bmatrix}t + \begin{bmatrix} 3 \\ 0 \\ 0 \end{bmatrix}##. To find the points on this line, you would need to go out 3 units on the x-axis and then go in the direction of (3t/7, t/7, t).
ChiralSuperfields
Homework Statement
Relevant Equations
For this problem,

The solution is,

However, I could not get that. By getting the system in REF, I got ##x = 3 + \frac{3}{7}z ## and ##y = \frac{1}{7}z##. Therefore z is a free variable so ##x = 3 + \frac{3}{7}t = 0## and ##y = \frac{1}{7}t##.

Thus equation of line is ##x\hat i + y\hat j + z\hat k = 3\hat i + (\frac{3}{7}\hat i + \frac{1}{7}\hat j)t##

Does anybody please know what I did wrong here?

Many thanks!

Last edited:
ChiralSuperfields said:

By getting the system in REF, I got x=3+37z=0 and y=17z.
You are right to get
$$x=3+\frac{3}{7}z$$
but why do you make it zero ?

ChiralSuperfields
anuttarasammyak said:
You are right to get
$$x=3+\frac{3}{7}z$$
but why do you make it zero ?

Sorry my mistake. That was not meant to be set to zero.

Many thanks!

ChiralSuperfields said:

For this problem,
View attachment 325740
The solution is,
View attachment 325741
However, I could not get that. By getting the system in REF, I got ##x = 3 + \frac{3}{7}z ## and ##y = \frac{1}{7}z##. Therefore z is a free variable so ##x = 3 + \frac{3}{7}t = 0## and ##y = \frac{1}{7}t##.
I got the same thing but took it one step further, with z = t. So the equation of the line of intersection is ##L = (\frac 3 7 t + 3)\hat i + \frac 1 7 t \hat j + t \hat k##.

In vector form, the above is ##L = \begin{bmatrix} \frac 3 7 \\ \frac 1 7 \\ 1 \end{bmatrix}t + \begin{bmatrix} 3 \\ 0 \\ 0 \end{bmatrix}##

Geometrically, to get points on this line, go out 3 units on the x-axis, and then go off in the direction of <3t/7, t/7, t>.
ChiralSuperfields said:
Thus equation of line is ##x\hat i + y\hat j + z\hat k = 3\hat i + (\frac{3}{7}\hat i + \frac{1}{7}\hat j)t##

ChiralSuperfields

• Calculus and Beyond Homework Help
Replies
9
Views
843
• Calculus and Beyond Homework Help
Replies
20
Views
549
• Calculus and Beyond Homework Help
Replies
8
Views
236
• Calculus and Beyond Homework Help
Replies
16
Views
1K
• Calculus and Beyond Homework Help
Replies
1
Views
677
• Calculus and Beyond Homework Help
Replies
1
Views
188
• Calculus and Beyond Homework Help
Replies
2
Views
484
• Calculus and Beyond Homework Help
Replies
5
Views
355
• Calculus and Beyond Homework Help
Replies
3
Views
1K
• Calculus and Beyond Homework Help
Replies
7
Views
734