Find the perimeter of a triangle with points A(1,2), B(1,8), and C(5,5) on a Cartesian coordinate plane.

Check the picture below.the distance from 1,2 to 1,8 is simply 6 units, we can read that off the grid.  Now let's see what the other distances are, and add them all up to get the perimeter.[tex]\bf ~~~~~~~~~~~~\textit{distance between 2 points} \\\\ A(\stackrel{x_1}{1}~,~\stackrel{y_1}{2})\qquad C(\stackrel{x_2}{5}~,~\stackrel{y_2}{5})\qquad \qquad d = \sqrt{( x_2- x_1)^2 + ( y_2- y_1)^2} \\\\\\ AC=\sqrt{(5-1)^2+(5-2)^2}\implies AC=\sqrt{4^2+3^2} \\\\\\ AC=\sqrt{25}\implies AC=5 \\\\[-0.35em] ~\dotfill[/tex][tex]\bf B(\stackrel{x_1}{1}~,~\stackrel{y_1}{8})\qquad C(\stackrel{x_2}{5}~,~\stackrel{y_2}{5}) \\\\\\ BC=\sqrt{(5-1)^2+(5-8)^2}\implies BC=\sqrt{4^2+3^2} \\\\\\ BC=\sqrt{25}\implies BC=5 \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ ~\hfill \stackrel{perimeter}{6+5+5\implies 16}~\hfill[/tex]