93. A cube is placed so that one comer is at the origin and three edges are along the $x-, y$, and $z$ axes of a coordinate system (Fig. 1.43). Use vectors to compute (a) the angle between the edge along the z-axis (line $a b$ ) and the diagonal from the origin to the opposite comer (line $a d$ ), and (b) the angle between line $a c$ (the diagonal of a face) and line ad.

 93. A cube is placed so that one comer is at the origin and three edges are along the $x-, y$, and $z$ axes of a coordinate system (Fig. 1.43). Use vectors to compute (a) the angle between the edge along the z-axis (line $a b$ ) and the diagonal from the origin to the opposite comer (line $a d$ ), and (b) the angle between line $a c$ (the diagonal of a face) and line ad.