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enter image description here

I don't understand that the pointed part(with a red circle) is a negative sign. I expect it should be a positive sign since the whole parenthesis was squared. I don't know where I got wrong.

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  • $\begingroup$ Pure econometrics questions without connection to economics should be posted on cross validated stack exchange. $\endgroup$
    – 1muflon1
    Sep 11 '20 at 17:09
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    $\begingroup$ It is a direct consequence of the three first lines (and orthogonality). $\endgroup$
    – Bertrand
    Sep 11 '20 at 18:58
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    $\begingroup$ @1muflon1: Actually, according to our policies, econometrics questions, including the theoretical ones, are on topic. $\endgroup$
    – Herr K.
    Sep 11 '20 at 21:20
  • $\begingroup$ @HerrK. Oh I seen in past pure econometric questions being directed towards cross validated but I will retract my close vote then thanks for letting me know $\endgroup$
    – 1muflon1
    Sep 11 '20 at 21:21

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