- #1

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[itex]\frac{d\hat{b}}{ds}=-\tau\hat{n}[/itex]

I understand that, [itex]\hat{t}\wedge\frac{d\hat{n}}{ds}\parallel\hat{n}[/itex] and [itex]|\frac{d\hat{b}}{ds}|=\tau[/itex], but why the negative sign? Isn't it equally possible that it has a positve sign?