Hi guys, attached is a picture of my problem and it is also underlined.(adsbygoogle = window.adsbygoogle || []).push({});

I've been reading through this theory and I just don't understand what the square brackets indicate.

I understand that ∇phi is the partial derivative with respect to the scalar function phi.

But what is ∇phi [r(t)] ?

I feel ashamed asking this like I'm going to be laughed at.

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# Not sure what square brackets indicate when dealing with partial derivates

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