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[tex]\frac{1}{\rho_c}=\frac{\partial G/\partial S}{2\sqrt{E} G}[/tex]

where [tex]s[/tex] is the arc length parameter and [tex]E[/tex], [tex]G[/tex] are the coefficents of the first fundamental form.

Can you please tell me how to perfrom the [tex]\partial G/\partial S[/tex]? Since [tex]G=r_v\cdot r_v[/tex] I am not sure how to derivate it with respect to arc length

Thanks!