# Prove any three vectors of R^3 are linear independent

1. Apr 17, 2004

### philipc

I was just wondering if I can make this statment, if I was to prove any three vectors of R^3 are linear independent, can I also say those three vectors span R^3?

Philip

2. Apr 18, 2004

### chroot

Staff Emeritus
No; a set of vectors can be linearly independent without spanning a space, and a set of vectors can span a space without being linearly independent. A set of vectors that is both linearly independent and spans a space is a basis for that space.

- Warren

3. Apr 18, 2004

### enigma

Staff Emeritus
Yes Warren, but it isn't possible to have more than three linearly independant vectors in R3, therefore any set of three will span and be a basis.

I think you just lost the forest for the trees

4. Apr 18, 2004

### philipc

Ok thanks for your help, can I ask one more, well maybe only one

I'm looking at my notes and kind of stuck with something,

Lets say I have the vectors [1,0] and [-1,1] and I'm asked what is the span?
so I have a[1,0] +b[-1,0] = [a-b,b] then it says this spans R^2 because [x,y] = [(x+y)-y,y] so x+y=a and y=b

not sure if I was sleeping during class and wrote the notes down wrong or what, but I'm not sure where the [x,y] = [(x+y)-y,y] came from?
Philip

5. Apr 18, 2004

### matt grime

You've shown how to write any vector in R^2 as a combination of [1,0] and [-1,1], therefore is spans. [x,y] = (x+y)[1,0] + y[-1,1]

6. Apr 18, 2004

### philipc

but it was not the work of me?

so is that saying I need to find values for a and b to make it = [x,y], and that can be any combo of x and y for the values of a and b? Sorry to sound so dumb, but not really sure if I'm getting it.
Philip

7. Apr 18, 2004

### matt grime

yep, you need to solve the vector equations a[1,0]+b[-1,1] = [x,y]

or splitting it into components

a-b=x
b=y

ie b=y, and a-y=x, a=x+y

8. Apr 18, 2004

### philipc

OK now I see where it came from, thanks for the help.
Philip

9. Apr 18, 2004

### HallsofIvy

Staff Emeritus
Any basis for a vector space has three properties:

1. It spans the space
2. The vectors in it are linearly independent
3. The number of vectors in it is the same as the dimension of the space.

If any two of those are true, then the third is also true.

If you know that a set of three vectors in R3 is independent, then the must span the space.

(Strictly speaking, we define a basis as a set of vectors satisfying (1) and (2), show that any basis must have the same number of vectors and then define the dimension of the space to be that number.)

Last edited: Apr 18, 2004