# Find inner product

#### Denis99

##### New member
Let $$\displaystyle <x, x>=3x_{1}^2+2x_{2}^2+x_{3}^2-4x_{1}x_{2}-2x_{1}x_{3}+2x_{2}x_{3}$$ be a quadratic form in V=R, where $$\displaystyle x=x_{1}e_{1}+x_{2}e_{2}+x_{3}e_{3}$$ (in the base $$\displaystyle {e_{1},e_{2},e_{3}}$$.
Find the inner product corresponding to this quadratic form.

Is this that easy that you have to change '' second'' x-es for y (for example to write $$\displaystyle 2x_{2}y_{3}$$ instead of $$\displaystyle 2x_{2}x_{3}$$ at the end), or what I have to do?

Last edited:

#### steep

##### Member
Let $$\displaystyle <x, x>=3x_{1}^2+2x_{2}^2+x_{3}^2-4x_{1}x_{2}-2x_{1}x_{3}+2x_{2}x_{3}$$ be a quadratic form in V=R, where $$\displaystyle x=x_{1}e_{1}+x_{2}e_{2}+x_{3}e_{3}$$ (in the base $$\displaystyle {e_{1},e_{2},e_{3}}$$.
Find the inner product corresponding to this quadratic form.

Is this that easy that you have to change '' second'' x-es for y (for example to write $$\displaystyle 2x_{2}y_{3}$$ instead of $$\displaystyle 2x_{2}x_{3}$$ at the end), or what I have to do?
what definition do you have to work with here? If it were me I'd iterate to the result-- start by encoding this with standard basis vectors, i.e.

$\langle x, x \rangle =3x_{1}^2+2x_{2}^2+x_{3}^2-4x_{1}x_{2}-2x_{1}x_{3}+2x_{2}x_{3} = \mathbf x^T A \mathbf x$

where $A$ is real symmetric positive definite. Then take the square root of $A$ and let that be your basis i.e. consider
$A^\frac{1}{2} \mathbf x$
where the $kth$ column of $A^\frac{1}{2}$ is denoted by $e_k$ in your text

#### Denis99

##### New member
what definition do you have to work with here? If it were me I'd iterate to the result-- start by encoding this with standard basis vectors, i.e.

$\langle x, x \rangle =3x_{1}^2+2x_{2}^2+x_{3}^2-4x_{1}x_{2}-2x_{1}x_{3}+2x_{2}x_{3} = \mathbf x^T A \mathbf x$

where $A$ is real symmetric positive definite. Then take the square root of $A$ and let that be your basis i.e. consider
$A^\frac{1}{2} \mathbf x$
where the $kth$ column of $A^\frac{1}{2}$ is denoted by $e_k$ in your text
I have to work with definition like this one from definition of inner space in here
https://en.m.wikipedia.org/wiki/Inner_product_space
(in part Definition)

#### steep

##### Member
I have to work with definition like this one from definition of inner space in here
https://en.m.wikipedia.org/wiki/Inner_product_space
(in part Definition)
But I can't figure out how you could understand that and say this

Find the inner product corresponding to this quadratic form.

Is this that easy that you have to change '' second'' x-es for y (for example to write $$\displaystyle 2x_{2}y_{3}$$ instead of $$\displaystyle 2x_{2}x_{3}$$ at the end), or what I have to do?
The reality is that one way or another you need to find $A^\frac{1}{2}$