Calculus 8th Edition

Neither, $cos^{-1}|\dfrac{-1}{\sqrt {11}}| \approx 95.2 ^{\circ}$
$n_1=\lt1,-1,3\gt$ and $n_2=\lt 3,1,-1\gt$ Angle between two vectors$\theta= cos^{-1}|\dfrac{a.b}{|a||b|}|$ $\theta= cos^{-1}|\dfrac{-1}{\sqrt {11} \sqrt {11}}|$ $\theta= cos^{-1}|\dfrac{-1}{\sqrt {11}}|$ $\theta \approx 95.2 ^{\circ}$ Hence, the two planes are neither parallel or perpendicular.