Orthogonality theorem proof method question

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SELFMADE

can't figure out how that underlined segment transforms into what's at the bottom:

[PLAIN]http://img7.imageshack.us/img7/9493/imag0254p.jpg

Last edited by a moderator: May 5, 2017

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Stephen Tashi

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It relies on
[tex]||x||^2 = x^T x[/tex]
[tex]||y||^2 = y^T y[/tex]
[tex](-x^T y - y^T x) = -x^T y - x^Ty = -2 x^T y[/tex]