Thank you for your reply! It was not necessary to go through the context.
But I didn't get the last thing:
why is it also dimensionless? It has direction and that's how we can find the sense of dτ. Sorry for stupid question (in case of it).
Hi! Here's a simple part of the lecture on mechanics (Fig.). And the trivial question is why the dα (which is in radian) equals dτ (which is vector), the idea of the question is why vector value could be equal to radian? Isn't it strange? Vector must contain three numbers, or maybe length and...