Page:Elements of the Differential and Integral Calculus - Granville - Revised.djvu/202



Center of curvature. If a circle be drawn through three points P0, P1, P2 on a plane curve, and if P1 and P2 be made to approach P0 along the curve as a limiting position, then the circle will in general approach in magnitude and position a limiting circle called the circle of curvature of the curve at the point P0. The center of this circle is called the center of curvature.

Let the equation of the curve be

and let $$x_0, x_1, x_2$$ be the abscissas of the points $$P_0, P_1, P_2$$ respectively, $$(\alpha', \beta')$$ the coördinates of the center, and $$R'$$ the radius of the circle passing through the three points. Then the equation of the circle is

and since the coordinates of the points $$P_0, P_1, P_2$$ must satisfy this equation, we have

Now consider the function of $$X$$ defined by

in which $$y$$ has been replaced by $$f(x)$$ from (1).

Then from equations (2) we get