(adsbygoogle = window.adsbygoogle || []).push({}); help me understand a line in an "ATA is positive, semi-definite" proof

I am looking at a proof for why ATA is positive semi-definite when A is nxn and it has this line.

vTAATv = ATv • ATv ≥ 0.

I understand what vTAATv means and the purpose of proving that it's nonnegative, etc... My problem is that I am a linear algebra novice and do not necessarily understand how the first part vTAATv is equivalent to ATv • ATv. I know that aTb = a • b, but something else is going on, no? Appreciate any help!

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# Help me understand a line in an ATA is positive, semi-definite proof

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