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sqrt(r^2-x^2) + l - sqrt(r^2 + d^2 - 2d*x) at x=r/(4d) (r+sqrt(r^2 + 8d^2))
sqrt(r^2-x^2) + l - sqrt(r^2 + d^2 - 2d*x) at x=r/(4d) (r+sqrt(r^2 + 8d^2))
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