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
If a matter distribution is spherically symmetric, the circular velocity is straightforwardly related to the mass via:
$v_{circ} = \sqrt{GM/r}$
In an axisymmetric potential, on the other hand hand, we have
$v_{circ}(R) = \sqrt{R\frac{d\Phi}{dR}}\neq \sqrt{GM/r}$
with $\Phi$ the gravitational ...