# find a vector in an orthnormal basis

• Sep 24th 2011, 12:27 PM
Kuma
find a vector in an orthnormal basis
suppose I have 3 vectors, a b and c that form an orthonormal basis. How would I find a vector v = (1,2,3) in the a b c system? I don't really understand what its asking. Do I have to find a linear combination from a b c that forms v?
so basically for some scalars x y and z, I have to find

xa + by + cz = v?

if I'm right, how would I go about doing that?
• Sep 24th 2011, 01:47 PM
Drexel28
Re: find a vector in an orthnormal basis
Quote:

Originally Posted by Kuma
suppose I have 3 vectors, a b and c that form an orthonormal basis. How would I find a vector v = (1,2,3) in the a b c system? I don't really understand what its asking. Do I have to find a linear combination from a b c that forms v?
so basically for some scalars x y and z, I have to find

xa + by + cz = v?

if I'm right, how would I go about doing that?

That's the beauty of the orthonormal bases. We know that $\displaystyle v=xa+by+cz$ for some $\displaystyle x,y,z\in\mathbb{C}$ but then $\displaystyle \langle v,a\rangle=x\langle a,a\rangle+y\langle b,a\rangle+z\langle c,a\rangle=x\cdot1 +y\cdot0+z\cdot 0$ and so $\displaystyle \langle v,a\rangle=x$. Doing the same analysis we arrive at the fact that $\displaystyle v=\langle v,a\rangle a+\langle v,b\rangle b+\langle v,c\rangle c$.
• Sep 24th 2011, 02:09 PM
Kuma
Re: find a vector in an orthnormal basis
wait I'm a little lost. How did x(a,a) turn into x? (a,a) is just ((a1,a2,a3),(a1,a2,a3)) in R3. So its just a set of vectors. How did it turn into 1? Unless I'm misunderstanding. What do the pointy brackets mean?
• Sep 24th 2011, 02:12 PM
Drexel28
Re: find a vector in an orthnormal basis
Quote:

Originally Posted by Kuma
wait I'm a little lost. How did x(a,a) turn into x? (a,a) is just ((a1,a2,a3),(a1,a2,a3)) in R3. So its just a set of vectors. How did it turn into 1? Unless I'm misunderstanding. What do the pointy brackets mean?

The angle brackets mean inner product, i.e. $\displaystyle \langle a,a\rangle=a\cdot a=1$. Make sense now?
• Sep 24th 2011, 02:19 PM
Kuma
Re: find a vector in an orthnormal basis
ahh ok. Thanks. Although I think I got the answer through row reduction before your post. Basically I made a matix with the vectors abc and set them equal to v. I got a unique scalar solution for x y and z Those scalars multiplied by a b and c gives me v.
• Sep 24th 2011, 02:20 PM
Drexel28
Re: find a vector in an orthnormal basis
Quote:

Originally Posted by Kuma
ahh ok. Thanks. Although I think I got the answer through row reduction before your post. Basically I made a matix with the vectors abc and set them equal to v. I got a unique scalar solution for x y and z Those scalars multiplied by a b and c gives me v.

That works too haha