Angle between A & B: 35.26 degrees or 90 degrees?

[jrevard]

This is from an advanced physics book, but is one of the first problems in the first chapter and only deals with vectors. The reported answer and mine do not coincide. For part (d) I get 35.26 degrees, the reported answer is 90 degrees. (Note: all variables are vectors)

Homework Statement

Consider a cube whose edges are each of unit length. One corner coincides with the origin of an xyz Cartesian coordinate system. Three of the cube's edges extend from the origin along the positive direction of each coordinate axis. Find the vector that begins at the origin and extends
(a) along a major diagonal of the cube;
(b) along the diagonal of the lower face of the cube.
(c) Calling these vectors A and B, find C = A x B.
(d) Find the angle between A and B

Homework Equations

cos(theta) = (A dot B)/(|A||B|)

The Attempt at a Solution

I get cos(theta) = 2/(root(6)),

they get cos(theta) = (1-1)/(root(6)) = 0

 

[SammyS]

From the wording of the problem, I would say that you are correct.

From the book's solution, it looks like they took the diagonal of the lower face that doesn't start (nor end) at the origin.