Find the acute angle between the following planes: `2x-y+z=5` and `x+y+2z=7` ii) `x+2y+2z=3` and `2x-3y+6z=8`

Answer

i) `pi/3`, ii) `cos^(-1)(8/21)`, iii) `cos^(-1)(sqrt(2)/3))`, iv) `cos^(-1)(2/(3sqrt(6)))`