PQ &amp;= \sqrt{(k-t)^2  + (h-r)^2}\\P^\prime Q^\prime &amp;= \sqrt{(h-r)^2  + (-k-(-t))^2} = \sqrt{(h-r)^2  + (t-k)^2}\\&amp;\qquad \qquad \qquad \qquad \qquad \qquad \quad = \sqrt{(k-t)^2 + (h-r)^2}