Math Is Fun Forum

Identity

Member

Registered: 2007-04-18

Posts: 934

Lagrange's identity

Can anyone help me proving that

thanks

simplyjasper

Member

Registered: 2009-11-15

Posts: 24

Re: Lagrange's identity

By Lagrange Identity,

Last edited by simplyjasper (2009-11-22 00:35:02)

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Lagrange's identity

Hi identity and simplyjasper;

That's what I am looking at but am having a little problem finishing up.

If you go here and about 1 / 4 of the page down you will find the paragraph "Lagrange's identity and vector calculus"

http://en.wikipedia.org/wiki/Lagrange%27s_identity

I am having a little trouble manipulating their identities to look like your problem.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Identity

Member

Registered: 2007-04-18

Posts: 934

Re: Lagrange's identity

Yes, that's also what I had trouble with. It was a homework question for a calculus class that I attended (but wasn't enrolled in). I don't think it should involve complicated algebra since it was only for first year calculus.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Lagrange's identity

Hi identity;

I think when you square the LHS of your problem you should get:

(a x b)²  I don't understand how they get the dot product in there (a x b) • (a x b)

Also look at this:

http://answers.yahoo.com/question/index … 850AA83WT7

Last edited by bobbym (2009-11-22 00:58:52)

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

simplyjasper

Member

Registered: 2009-11-15

Posts: 24

Re: Lagrange's identity

Isn't it just by using the identity and taking away from both side
(a . b)^2

Or you need to prove the identity before you can use it?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Lagrange's identity

Hi simplyjasper;

Yes, that would work, unless they want him to prove that identity that you are quoting.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Identity

Member

Registered: 2007-04-18

Posts: 934

Re: Lagrange's identity

Thanks bobbym, that's what I was looking for