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July 29th, 2014, 11:21 AM  #1 
Member Joined: Mar 2013 Posts: 71 Thanks: 4  prove that $x$ and $y$ are multiples
Hellow! I've got the following problem: "Let $x,y \in \mathbb R^n$. If every $z \in \mathbb R^n$ which is orthogonal to $x$ is also orthogonal to $y$, prove that $x$ and $y$ are multiples". Proof: consider two vectors $x,y$. The projection of $y$ into $x$ is $w=\frac{<x,y>x}{<x,x>}$. Then we have $<yw,x>=0$. By hypothesis of the problem, $<yw,y>=0 \Rightarrow y^2=<w,y>$. If I could get that $x^2y^2=<x,y>^2$, it's done. But how? 
July 29th, 2014, 02:15 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,661 Thanks: 648 
Let y = ax + z, where (x,z) = 0, with a = (x,y)/(x,x). However, since (x,z) = 0, then (y,z) = 0. Therefore (y,y) = [(x,y)]^2/(x,x), which is what you are trying to show.


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$x$, $y$, multiples, prove 
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