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h(u,v) = H - S*sqrt((x-u)**2 + (y-v)**2)

h1 = h(1,0) = H - S*(1 - x)
h2 = h(0,1) = H - S*sqrt(1 + x*x)
h3 = h(-1,0) = H - S*(1 + x)

-h3 + h1 = S*(1 + x) - S*(1 - x) = 2*S*x
-h3 + h2 = S*(1 + x) - S*sqrt(1 + x*x)
-h3 + h3 = 0

2 * (-h3 + h1) / (h1 - h3) = 2
2 * (-h3 + h2) / (h1 - h3) = (1 + x - sqrt(1 + x*x)) / x
                           = 1/x + 1 - sqrt(1/x**2 + 1)

-1/x**2 + 1/(x**3 * sqrt(1 + 1/x**2))