by Guest » Thu Jan 30, 2025 11:03 am
let the coordinates of all three points be [tex](x_i,y_i,z_i)[/tex] where [tex]i=1,2,3[/tex] such that the third point is external to the line. Now, the parametric representation of any general point on the line is [tex]P(u)=\begin{cases} x=x_1+u(x_2-x_1) \\ y = y_1+u(y_2-y_1) \\ z=z_1+u(z_2-z_1) \end{cases}[/tex], where [tex]u\in[0,1][/tex]. Now, the distance from the third point to this general point is [tex]\sqrt{\left\{x_3-x_1-u(x_2-x_1)\right\}^2+\left\{y_3-y_1-u(y_2-y_1)\right\}^2+\left\{z_3-z_1-u(z_2-z_1)\right\}^2}[/tex].
So, taking the average over the entire interval, we get
[tex]\frac{1}{1-0}\int\limits_{0}^{1}\sqrt{\left\{x_3-x_1-u(x_2-x_1)\right\}^2+\left\{y_3-y_1-u(y_2-y_1)\right\}^2+\left\{z_3-z_1-u(z_2-z_1)\right\}^2}\,du[/tex]
[tex]=\boxed{\int\limits_{0}^{1}\sqrt{\left\{x_3-x_1-u(x_2-x_1)\right\}^2+\left\{y_3-y_1-u(y_2-y_1)\right\}^2+\left\{z_3-z_1-u(z_2-z_1)\right\}^2}\,du}[/tex]