In DoCarmo's Differential Geometry book there's an excerpt (on page 19-20) like this:

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Fundamental theorem of the local theory of curves:

Given differentiable functions and where , there exists a regular parametrized curve

such that is the arc length, is the curvature, and is the torsion of . Moreover, any other curve , satisfying the

same conditions, differs from by a rigid motion; that is, there exists an orthogonal linear map of , with positive determinant,

and a vector such that .

.........

Proof of the Uniqueness Part of the Fundamental Theorem:

We first remark that arc length, curvature, and torsion are invariant under rigid motions; that means, for instance, that if

is a rigid motion and is a parametrized curve, then

That is plausible, since these concepts are defined by using inner or vector products of certain derivatives (the derivatives are invariant under translations, and the

inner and vector products are expressed by means of lengths and angles of vectors, and thus also invariant under rigid motions).

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My question is:

why's that

?

How did the author prove this? Is it possible to help me prove this? I'm completely lost at the last line of the excerpt given above.