I'm reading the wikipedia page https://en.m.wikipedia.org/wiki/Menger_curvature
It mensions a so-called well-known formula here but doesn't mention its name. I guess it is heron's formula. But again it is very different from this formula. Heron's formula is $A=\sqrt{s(s-a)(s-b)(s-c)}$, where $s=\dfrac{a+b+c}2$. But it contains a square root and has no relationship with the radius R. I wonder which formula of the radius R it uses here to deduce this formula of curvature?