# Linear Algebra - independent vectors spanning R^3

## Homework Statement

Suppose that S={u,v,w} is a basis of R3.
a) Is {u-v,u+v} linearly independent? Why or why not?
b) Does {u+v,u-v} span R3? Why or why not?

na

## The Attempt at a Solution

a) Because we know that u,v,w are linearly independent and span R3, c1u+c2v+c3w = 0.
To test u+v, u-v, consider a(u+v)+b(u-v)=0. So, au+av+bu-bv=0, so u(a+b)+v(a-b)=0. Since u and v are linearly independent, a+b=0 and a-b=0. So 2b=0, so b=0, so a=0. So u+v, u-v are linearly independent.
b) Because there are only 2 vectors, u and v, the rank can only be 2, so it cannot span R3.

I'm pretty sure I got a) correct, but I'm not sure about b). Thanks!

Mark44
Mentor

## Homework Statement

Suppose that S={u,v,w} is a basis of R3.
a) Is {u-v,u+v} linearly independent? Why or why not?
b) Does {u+v,u-v} span R3? Why or why not?

na

## The Attempt at a Solution

a) Because we know that u,v,w are linearly independent and span R3, c1u+c2v+c3w = 0.
To test u+v, u-v, consider a(u+v)+b(u-v)=0. So, au+av+bu-bv=0, so u(a+b)+v(a-b)=0. Since u and v are linearly independent, a+b=0 and a-b=0. So 2b=0, so b=0, so a=0. So u+v, u-v are linearly independent.
b) Because there are only 2 vectors, u and v, the rank can only be 2, so it cannot span R3.

I'm pretty sure I got a) correct, but I'm not sure about b). Thanks!
Both a and b are right.

vela
Staff Emeritus