## Calculus 8th Edition

$cos ^{-1}(\frac{1}{ \sqrt 3} )\approx 55 ^ \circ$
Consider a cube with side length $1$ , one diagonal goes from $(0,0,0)$ to $(1,1,1)$ . An edge goes from $(0,0,0)$ to $(0,0,1)$ . $\theta = cos ^{-1}(\frac{\lt 1,1,1 \gt \cdot \lt 0,0,1 \gt}{\sqrt {1^2+1^2+1^2}\sqrt {0^2+0^2+0^2}})$ $\theta = cos ^{-1}(\frac{1}{ \sqrt 3} )\approx 55 ^ \circ$