# Thread: inner product not by standart basis

1. ## inner product not by standart basis

we choose a basis B={v1=(1,0)v2=(1,1)} from $R^2$
and define $(x,y)_B=x_1y_1+x_2y_2$

why does the prof say that
$x=x_1v_1+x_2v_2$
$y=y_1v_1+y_2v_2$
how did he get those formulas?
and i cant se the structure of our chosen basis here?
how to see it in thos formulas
?

2. ## Re: inner product not by standart basis

Originally Posted by transgalactic
we choose a basis B={v1=(1,0)v2=(1,1)} from $R^2$ and define $(x,y)_B=x_1y_1+x_2y_2$ why does the prof say that $x=x_1v_1+x_2v_2$ $y=y_1v_1+y_2v_2$
Although I don't know what is the final question, taking into account that $B$ is a basis of $\mathbb{R}^2$ , every $x\in \mathbb{R}^2$ can be uniquely expressed in the way $x=x_1v_1+x_2v_2$ . Same considerations for $y$ .

3. ## Re: inner product not by standart basis

if x=(x1,x2) then x could be represented as $x=x_1v_1+x_2v_2$

only if v1=(1,0) v2=(0,1)

but here we have a different basis not a standart one.
so our coefficient is not x1 x2

??

4. ## Re: inner product not by standart basis

Perhaps this simple example will help you. Consider $x=(5,2)$ and $y=(2,1)$ . Prove that $x=3v_1+2v_2$ and $y=1v_1+1v_2$ . According to the given definition of the inner product we have $(x|y)=3\cdot 1+2\cdot 1=5$ .

5. ## Re: inner product not by standart basis

ok so if i will go by your example
(x,y)_B=x1y1+x2y2=5*2+2*1=12

x=(5,2)=5(1,0)+2(1,1)=(7,2)

i try to follow the logic but i cant understand this term

6. ## Re: inner product not by standart basis

Originally Posted by transgalactic
x=(5,2)=5(1,0)+2(1,1)=(7,2)
$(5,2)\neq (7,2)$

7. ## Re: inner product not by standart basis

i know
i got here some thing wrong
just cant understand this basis of inner product
i am looking for a tutorial with basic examples