# Prove a subset

judahs_lion

## Homework Statement

Show that W ={(x,y,z) : x +2y +3z =0} is a subspace of R3. By finding a subset S of W such that span(S) = W.

Ab = X?

## The Attempt at a Solution

I don't have an attempt because i'm completely lost where to start . Can someone point me in the right way please.

Last edited:

Homework Helper
Start with the definition of a subspace. Obviously every element of W is in R^3, so W is a subset (set!) of R^3. Now, what is the easiest subspace "test"?

judahs_lion
Start with the definition of a subspace. Obviously every element of W is in R^3, so W is a subset (set!) of R^3. Now, what is the easiest subspace "test"?

So prove that is its

Independent
Closed Under Scalar Multiplication?

How to I find S?

Staff Emeritus
Gold Member
The equation defines a plane, so you need to find two linearly independent vectors in that plane.

judahs_lion
Is this system correct?

(x, y, z) = y(-2,1,0) +z(-3,0,1)

So S would be <(-2,1,0),(-3,0,1)>

Staff Emeritus
Gold Member
Yes, that appears to be correct. But I don't like the <,> notation. Does your book or teacher write the span that way?

Mentor
Is this system correct?

(x, y, z) = y(-2,1,0) +z(-3,0,1)

So S would be <(-2,1,0),(-3,0,1)>

Those vectors work. I would write it as S = {<-2, 1, 0>, <-3, 0, 1>}. This set of vectors spans W. Now what you need to do is to show that W is a subspace of R3.

Something you said earlier in this thread deserves comment:
judahs_lion said:
So prove that is its

Independent
Closed Under Scalar Multiplication?

The first one you listed is not a test for showing that a set is a subspace of some vector space. The vectors in a basis have to linearly independent, and have to span some subspace.

What is the test you missed?

judahs_lion
Those vectors work. I would write it as S = {<-2, 1, 0>, <-3, 0, 1>}. This set of vectors spans W. Now what you need to do is to show that W is a subspace of R3.

Something you said earlier in this thread deserves comment:

The first one you listed is not a test for showing that a set is a subspace of some vector space. The vectors in a basis have to linearly independent, and have to span some subspace.

What is the test you missed?

Contains the zero vector?

judahs_lion
How do I test for the zero vector?

Mentor
Does W contain the zero vector? It's a very easy test.

judahs_lion
Does W contain the zero vector? It's a very easy test.

I was think of testing S for zero vector.

So for (x, y, x) =(0,0,0)

0 + 2(0) +3(0) = 0 Checks out.

Proper test? So for being so needy, math is a weak point of mine.

Mentor
Sure, that's another way to do it.

Your set W represents a plane in three-dimensional space. This plane contains the origin, which you can tell by noticing that (0, 0, 0) is a solution of the equation x + 2y + 3z = 0.

Set S contains two vectors that lie in the plane. It doesn't contain the zero vector. You want to test set W to see whether it (W) contains the zero vector.

Since there are two vectors, and neither is a multiple of the other, they are linearly independent. Since the plane is two dimensional, and you have two linearly independent vectors, the two vectors span the plane, which means that every vector in the plane is a linear combination of the vectors you found. Instead of noticing that (0, 0, 0) is a point on this plane, you instead found that 0*<-2, 1, 0> + 0*<-3, 0, 1> = <0, 0, 0>, which says that the zero vector is in W (or equivalently, that (0,0,0) is on the plane).

judahs_lion
Thank you

judahs_lion
the matrix of { 1+x^2, x +x^3} = [1,0;0,1;1,0;0,1] ?

Mentor
the matrix of { 1+x^2, x +x^3} = [1,0;0,1;1,0;0,1] ?
What's the actual question?

Staff Emeritus