# Homework Help: Projections onto subspaces

1. Nov 23, 2011

### shaon0

1. The problem statement, all variables and given/known data
See attachment

3. The attempt at a solution
How should I approach these questions? By using the projection formula?

#### Attached Files:

• ###### Projections onto planes.png
File size:
13.6 KB
Views:
151
2. Nov 23, 2011

### HallsofIvy

Yes, since the problem asks for projections, use the projection formula!

3. Nov 23, 2011

### shaon0

Will I have to do two different calculations ie one with v and the first vector in W and then v with the second vector in W? Also, how would I find P?

4. Nov 24, 2011

### vela

Staff Emeritus
W is a plane. The question is asking you to find the projection of the vector v onto that plane.

As far as finding the matrix P goes, the projection onto W is a linear mapping. How do you find the matrix representing a linear mapping?

5. Nov 25, 2011

### shaon0

For P, the projection of what vector onto W? Would I just span the vectors i've found so, P=span{v1,v2} and find the co-effs s.t Basis1=a1.v1+a2.v2 where v1,v2 are the vectors i've found using the projection formula and a1,a2 are constants which will give me the 1st column of P?

Last edited: Nov 25, 2011
6. Nov 25, 2011

### vela

Staff Emeritus
You might find the page helpful:

http://www.cliffsnotes.com/study_guide/Projection-onto-a-Subspace.topicArticleId-20807,articleId-20792.html [Broken]

Last edited by a moderator: May 5, 2017
7. Nov 30, 2011

### shaon0

I've tried to use the projWv=projv1v+projv2v where v1=(-2,,1,-2)^T and v2=(1,4,-8)^T but i don't get the correct answer. Maybe because v1, v2 are not mutually orthogonal?

Last edited by a moderator: May 5, 2017
8. Nov 30, 2011

### unlearned

Since any vector $v$ can be written as a linear combination of vectors of a basis of $\mathbb{R}^3$, if you can find the projection of each of the vectors of the canonical basis of $\mathbb{R}^3$, you can then write a projection matrix using the results.

9. Nov 30, 2011

### vela

Staff Emeritus
Yes, that's why it didn't work.

10. Dec 1, 2011

### shaon0

So; proje1v1? for all combinations of e1,e2,e3 and v1, v2?

11. Dec 1, 2011

### vela

Staff Emeritus
If you look at the first picture on that web page, it illustrates that you can write
$$\vec{v} = (\mathrm{proj}_W\ \vec{v}) + \vec{v}_\perp$$That is, you can resolve any vector $\vec{v}$ into a piece that lies in the plane W and a piece that's perpendicular to W. Solving for the projection, you get
$$\mathrm{proj}_W\ \vec{v} = \vec{v} - \vec{v}_\perp$$So if you can figure out how to find $\vec{v}_\perp$, which is very likely a problem you solved before, you can then find the projection of $\vec{v}$ onto W. Hint: think about the normal to the plane.

If you don't want to use that approach, you can go with the method you tried. But as you noted, you need an orthogonal basis for W. You've been given a basis. You just need to make it orthogonal.

12. Dec 2, 2011

### shaon0

Ok, thanks. I think i've got it.