# Determining if two planes are parallel with variables?

• February 9th 2013, 11:57 AM
wavingguy
Determining if two planes are parallel with variables?
I've been given two planes. P1 = 2x + ay + 4z = 8 and P2 = ax + 4y +8z = 18

First I was asked to find the normal vectors of both planes:

I found N1 = (2,a,4) and N2 = (a,4,8)

Then I was asked "For what values of 'a' will the two planes be parallel?"

This is where I'm stuck, I cannot find a value of 'a' where the two planes are parallel.
• February 9th 2013, 12:00 PM
Plato
Re: Determining if two planes are parallel with variables?
Quote:

Originally Posted by wavingguy
I've been given two planes. P1 = 2x + ay + 4z = 8 and P2 = ax + 4y +8z = 18
First I was asked to find the normal vectors of both planes:
I found N1 = (2,a,4) and N2 = (a,4,8)
Then I was asked "For what values of 'a' will the two planes be parallel?"
This is where I'm stuck, I cannot find a value of 'a' where the two planes are parallel.

Two planes are parallel if their normals are parallel, ie $N_1\|N_2$.
• February 9th 2013, 12:05 PM
wavingguy
Re: Determining if two planes are parallel with variables?
I understand that, but with the normals I've found there is not value of 'a' that will make the planes parallel. Is it possible that the planes are never parallel?
• February 9th 2013, 12:09 PM
Plato
Re: Determining if two planes are parallel with variables?
Quote:

Originally Posted by wavingguy
I understand that, but with the normals I've found there is not value of 'a' that will make the planes parallel. Is it possible that the planes are never parallel?

Well then that is the answer. They cannot be parallel.