Find the angle between the planes x + y + z = 1 and x - 2y + 3z = 1.
1
\(\rm \cos^{-1}\left(\sqrt{\frac{1}{42}}\right)\)
2
\(\rm \cos^{-1}\left(\sqrt{\frac{2}{42}}\right)\)
3
\(\rm \cos^{-1}\left(\sqrt{\frac{2}{21}}\right) \)
4
\(\rm \cos^{-1}\left(\sqrt{\frac{1}{7}}\right)\)