Mathematics

Problem

Show that the three lines with direction cosines

\frac{12}{13}, \frac{-3}{13}, \frac{-4}{13}

;

\frac{4}{13}, \frac{12}{13}, \frac{3}{13}

;

\frac{3}{13}, \frac{-4}{13}, \frac{12}{13}

are mutually perpendicular.