Find the acute angle between the following planes: i) `vecr.(hati+hatj-2hatk)=5` and `vecr.(2hati+2hatj-hatk)=9` ii) `vecr.(hati+2hatj-hatk)=6` and `vecr.(2hati-hatj-hatk)+3=0`

Answer

i) `cos^(-1)(sqrt(6)/3)`, ii) `cos^(-1)(1/6)`, iv) `cos^(-1)(6/7)`